[llvm] [CodeGen] commuteInstruction should update implicit-def (PR #131361)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> What would be the alternative for subreg-to-reg to describe zeroing of the other lanes in the destination register? 

We do not need an alternative. This is a completely unused feature. The targets just need to explicitly write the full super-register that may be used in the selected instructions, and use contexts using a subset have an explicit extract 

https://github.com/llvm/llvm-project/pull/131361