[llvm] [AMDGPU] Fuse dword load + sign/zero-extension into subword load (PR #219019)

[Krzysztof Drewniak via llvm-commits]

https://github.com/krzysz00 commented:

This does look correct ... but also, doesn't it apply to non `s_*` instructions -`ds_read_u8`, `global_load_i16`, and so on?

https://github.com/llvm/llvm-project/pull/219019