[llvm] [AMDGPU] Support TFE D16 format buffer loads on gfx8/10/11/12 (PR #211465)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

The globalisel legalizer stuff is still ugly, we need better APIs 

https://github.com/llvm/llvm-project/pull/211465