These date back to when the non-intrinsic format of variable locations
was still being tested and was behind a compile-time flag, so not all
builds / bots would correctly run them. The solution at the time, to get
at least some test coverage, was to have tests opt-in to non-intrinsic
debug-info if it was built into LLVM.

Nowadays, non-intrinsic format is the default and has been on for more
than a year, there's no need for this flag to exist.

(I've downgraded the flag from "try" to explicitly requesting
non-intrinsic format in some places, so that we can deal with tests that
are explicitly about non-intrinsic format in their own commit).

The `poison` values are used to substitute debug information of values
moved from the original header into the preheader that are no longer
available in the former.

This PR removes tests with `br i1 undef` under
`llvm/tests/Transforms/Loop*, Lower*`.

The main use for this patch is LTO. It is not (yet?) possible to set the
size level (-Os, -Oz) in the linker, which means loops are still rotated
even if -Oz is specified on the command line. Therefore, look at the
function attribute instead of only at the size level to determine
whether to rotate loops for a given function.

For discussion, see: https://reviews.llvm.org/D119342

An older version of this patch was already approved at
https://reviews.llvm.org/D119342 but I never got around to committing
it. The code changed so I had to make some minor updates to this patch
and in the meantime I also lost commit access because I wasn't really
using it. So here is an updated patch.

This change builds on 0a357ad which supported non-constant strides in
howFarToZero, but used only context insensitive reasoning.

This change does two things:
1) Directly use context sensitive queries to prove facts established
   before the loop.  Note that we technically only need facts known
   at the latch, but using facts known on entry is a conservative
   approximation which will cover most everything.
2) For the non-zero check, we can usually prove non-zero from the
   finite assumption implied by mustprogress.  This eliminates the
   need to do the context sensitive query in the common case.

This patch makes the final major change of the RemoveDIs project, changing the
default IR output from debug intrinsics to debug records. This is expected to
break a large number of tests: every single one that tests for uses or
declarations of debug intrinsics and does not explicitly disable writing
records. 

If this patch has broken your downstream tests (or upstream tests on a
configuration I wasn't able to run):
1. If you need to immediately unblock a build, pass
`--write-experimental-debuginfo=false` to LLVM's option processing for all
failing tests (remember to use `-mllvm` for clang/flang to forward arguments to
LLVM).
2. For most test failures, the changes are trivial and mechanical, enough that
they can be done by script; see the migration guide for a guide on how to do
this: https://llvm.org/docs/RemoveDIsDebugInfo.html#test-updates
3. If any tests fail for reasons other than FileCheck check lines that need
updating, such as assertion failures, that is most likely a real bug with this
patch and should be reported as such.

For more information, see the recent PSA:
https://discourse.llvm.org/t/psa-ir-output-changing-from-debug-intrinsics-to-debug-records/79578

When analyzing pass debug output it is helpful to have the function name
along with the loop name.

Branch weight from sample-based PGO may be not inaccurate due to
sampling. If the loop body must be executed, then origin loop back
edge weight must be not less than exit weight.

Branch weight from sample-based PGO may be not inaccurate due to
sampling. This test tracks such case where updateBranchWeights wraps
unsigned.