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

[Matt Arsenault via llvm-branch-commits]

https://github.com/arsenm requested changes to this pull request.

The only if subreg liveness is enabled point is invalid. Downstream users are still invalid, it just happens subregister liveness disabled hides practical issues 

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