CodeGen: Record MMOs in finalizeBundle (#166210)
This allows more accurate alias analysis to apply at the bundle level.
This has a bunch of minor effects in post-RA scheduling that look mostly
beneficial to me, all of them in AMDGPU (the Thumb2 change is cosmetic).
The pre-existing (and unchanged) test in
CodeGen/MIR/AMDGPU/custom-pseudo-source-values.ll tests that MIR with a
bundle with MMOs can be parsed successfully.
v2:
- use cloneMergedMemRefs
- add another test to explicitly check the MMO bundling behavior
v3:
- use poison instead of undef to initialize the global variable in the
test
