fbf6271c7da20356d7b34583b3711b4126ca1dbb introduced an assertion failure as
setDebugValueUndef was called on DBG_LABELs, which isn't allowed and doesn't
make sense. Fix by skipping the call for DBG_LABELs and hoisting, in line with
the original behaviour.

Reapply #140091.

branch-folder hoists common instructions from TBB and FBB into their
pred. Without this patch it achieves this by splicing the instructions from TBB
and deleting the common ones in FBB. That moves the debug locations and debug
instructions from TBB into the pred without modification, which is not
ideal. Debug locations are handled in #140063.

This patch handles debug instructions - in the simplest way possible, which is
to just kill (undef) them. We kill and hoist the ones in FBB as well as TBB
because otherwise the fact there's an assignment on the code path is deleted
(which might lead to a prior location extending further than it should).

There's possibly something we could do to preserve some variable locations in
some cases, but this is the easiest not-incorrect thing to do.

Note I had to replace the constant DBG_VALUEs to use registers in the test- it
turns out setDebugValueUndef doesn't undef constant DBG_VALUEs... which feels
wrong to me, but isn't something I want to touch right now.

---

Fix end-iterator-dereference and add test.

Reverts llvm/llvm-project#149999

https://lab.llvm.org/buildbot/#/builders/139/builds/17622

Reapply #140091.

branch-folder hoists common instructions from TBB and FBB into their
pred. Without this patch it achieves this by splicing the instructions from TBB
and deleting the common ones in FBB. That moves the debug locations and debug
instructions from TBB into the pred without modification, which is not
ideal. Debug locations are handled in #140063.

This patch handles debug instructions - in the simplest way possible, which is
to just kill (undef) them. We kill and hoist the ones in FBB as well as TBB
because otherwise the fact there's an assignment on the code path is deleted
(which might lead to a prior location extending further than it should).

There's possibly something we could do to preserve some variable locations in
some cases, but this is the easiest not-incorrect thing to do.

Note I had to replace the constant DBG_VALUEs to use registers in the test- it
turns out setDebugValueUndef doesn't undef constant DBG_VALUEs... which feels
wrong to me, but isn't something I want to touch right now.

Reverts llvm/llvm-project#140091 due to crash (see comments for reproducer)

branch-folder hoists common instructions from TBB and FBB into their
pred. Without this patch it achieves this by splicing the instructions from TBB
and deleting the common ones in FBB. That moves the debug locations and debug
instructions from TBB into the pred without modification, which is not
ideal. Debug locations are handled in pull request 140063.

This patch handles debug instructions - in the simplest way possible, which is
to just kill (undef) them. We kill and hoist the ones in FBB as well as TBB
because otherwise the fact there's an assignment on the code path is deleted
(which might lead to a prior location extending further than it should).

We might be able to do something smarter to preserve some variable locations in
some cases, but this is the easiest not-incorrect thing to do.

This patch is part of a series that adds origin-tracking to the debugify
source location coverage checks, allowing us to report symbolized stack
traces of the point where missing source locations appear.

This patch adds the logic for collecting stack traces in DebugLoc
instances. We do not symbolize the stack traces in this patch - that
only happens when we decide to actually print them, which will be the
responsibility of debugify. The collection happens in the constructor of
a DebugLoc that has neither a valid location nor an annotation; we also
collect an extra stack trace every time we call setDebugLoc, as
sometimes the more interesting point is not where the DebugLoc was
constructed, but where it was applied to an instruction. This takes the
form of a getCopied() method on DebugLoc, which is the identity function
in normal builds, but adds an extra stack trace in origin-tracking
builds.

Part of the coverage-tracking feature, following #107279.

In order for DebugLoc coverage testing to work, we firstly have to set
annotations for intentionally-empty DebugLocs, and secondly we have to
ensure that we do not drop these annotations as we propagate DebugLocs
throughout compilation. As the annotations exist as part of the DebugLoc
class, and not the underlying DILocation, they will not survive a
DebugLoc->DILocation->DebugLoc roundtrip. Therefore this patch modifies
a number of places in the compiler to propagate DebugLocs directly
rather than via the underlying DILocation. This has no effect on the
output of normal builds; it only ensures that during coverage builds, we
do not drop incorrectly annotations and therefore create false
positives.

The bulk of these changes are in replacing
DILocation::getMergedLocation(s) with a DebugLoc equivalent, and in
changing the IRBuilder to store a DebugLoc directly rather than storing
DILocations in its general Metadata array. We also use a new function,
`DebugLoc::orElse`, which selects the "best" DebugLoc out of a pair
(valid location > annotated > empty), preferring the current DebugLoc on
a tie - this encapsulates the existing behaviour at a few sites where we
_may_ assign a DebugLoc to an existing instruction, while extending the
logic to handle annotation DebugLocs at the same time.

Add per-property has<Prop>/set<Prop>/reset<Prop> functions to
MachineFunctionProperties.

Assertion failure introduced in #140063, which didn't account for TBB
and FBB being the same block.