This verifier tries to ensure that DebugLoc's don't just disappear as
we transform the MIR. It observes the instructions created, erased, and
changed and at checkpoints chosen by the client algorithm verifies the
locations affected by those changes.
In particular, it verifies that:
- Every DebugLoc for an erased/changing instruction is still present on at least one new/changed instruction
- Failing that, that there is a line-0 location in the new/changed instructions. It's not possible to confirm which locations were merged so it conservatively assumes all unaccounted for locations are accounted for by any line-0 location to avoid false positives.
If that fails, it prints the lost locations in the debug output along with
the instructions that should have accounted for them.
In theory, this is usable by the legalizer, combiner, selector and any other
pass that performs incremental changes to the MIR. However, it has so far
only really been tested on the legalizer (not including the artifact
combiner) where it has caught lots of lost locations, particularly in Custom
legalizations. The tests in this patch are those the verifier detected a problem in after applying it to all the .mir tests for AArch64 and AMDGPU.