This addresses a performance issue for our downstream GPU target that
sets requiresStructuredCFG to true. The issue is that EarlyMachineLICM
pass does not hoist loop invariants because a critical edge is not
split.
The critical edge's destination a loop header. Splitting the critical
edge will not break structured CFG.

Add a nvptx test to demonstrate the issue since the target also
requires structured CFG.

---------

Co-authored-by: Matt Arsenault <arsenm2@gmail.com>

Post-RA machine sinking could sink a copy of sub-register into
a successor. However, the sub-register might not be removed from the
live-in bitmask of its super register in successor and then a later
pass, e.g, if-converter, may add an implicit use of the register from
live-in resulting in an use of an undefined register. This change makes
sure subrange of live-ins from super register could be removed as well.

These liveins are not defined by predecessors, so should not be 
considered as liveouts in predecessor blocks. This resolves:

- https://github.com/llvm/llvm-project/pull/149062#discussion_r2285072001
- https://github.com/llvm/llvm-project/pull/153417#issuecomment-3199972351

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

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.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

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

- Move `MIPrinter` class to anonymous namespace, and remove it as a
friend of `MachineBasicBlock`.
- Move `canPredictBranchProbabilities` to `MachineBasicBlock` and change
it to use the new `BranchProbability::normalizeProbabilities` function
that accepts a range, and also to use `llvm::equal()` to check equality
of the two vectors.
- Use `ListSeparator` to print comma separate lists instead of manual
code to do that.

This change splits out the renaming and comment updates from #129612 as a non-functional change.