[llvm] [AMDGPU] Delete dead meta instructions (PR #193030)

[via llvm-commits]

LU-JOHN wrote:

> Can we turn these IMPLICIT_DEFs into implicit-defs on the first terminator instruction?

If  we convert:
```
bb.parent:
  S_CBRANCH %bb.mid
bb.mid:
  IMPLICIT_DEF
  S_BRANCH %bb.child
```
  to:
```
bb.parent:
  S_CBRANCH %bb.mid
bb.mid:
  S_BRANCH %bb.child, implicit-def
```
branch-folder still will not be able to redirect bb.parent's branch to bb.child, because this would remove an implicit-def from the path.  

https://github.com/llvm/llvm-project/pull/193030