[llvm] [LiveVariables] Mark use as implicit-def if def is a subregister (PR #119446)

[via llvm-commits]

jofrn wrote:

> What do you need this for? We should be trying to get rid of the LiveVariables pass. All of the code here for handling sub registers is very out of date

If we do not have this, then `LiveRangeCalc::findReachingDefs` may fail:
```
Use of $sgpr2_sgpr3 does not have a corresponding definition on every path:
248r $sgpr3 = S_MOV_B32 0, implicit-def $sgpr0_sgpr1_sgpr2_sgpr3, implicit $sgpr0, implicit $sgpr1, implicit $sgpr2, implicit $sgpr0_sgpr1, implicit $sgpr0_sgpr1_sgpr2, implicit $sgpr2_sgpr3
LLVM ERROR: Use not jointly dominated by defs.
```

https://github.com/llvm/llvm-project/pull/119446