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

[Sander de Smalen via llvm-commits]

https://github.com/sdesmalen-arm commented:

> We really should get rid of subreg_to_reg. This hack is just going to spread everywhere.

What would be the alternative for subreg-to-reg to describe zeroing of the other lanes in the destination register? To me the hack is more that we want to describe this using a COPY with an `implicit-def`, especially when the implicit-def is a virtual register. I wonder if we would benefit from having something like:
```
zero %0.sub32:gpr64 = INST ...
```

to specify that the other lanes in the register are zeroed (similar to how `undef` can be used here)

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