[llvm] [AMDGPU] Allow shinking instruction with dead sdst (PR #68028)

[Carl Ritson via llvm-commits]

perlfu wrote:

For GFX, I quickly tried this on Navi21 shaders and saw no shift in _e32 versus _e64.  But I might have missed something.

https://github.com/llvm/llvm-project/pull/68028