[llvm-branch-commits] [llvm] Reland "RegisterCoalescer: Add implicit-def of super register when coalescing SUBREG_TO_REG" (PR #134408)

[Sander de Smalen via llvm-branch-commits]

sdesmalen-arm wrote:

Gentle ping @arsenm and @qcolombet 

I know that @arsenm is in favour of moving away from `SUBREG_TO_REG` entirely, but at the moment it is still used in many places by multiple targets and this PR fixes a genuine bug that is exposed with sub-reg liveness tracking.

https://github.com/llvm/llvm-project/pull/134408