[llvm] [AMDGPU][True16][CodeGen] use vgpr16 for zext patterns (reopen #153894) (PR #154211)

[Jan Patrick Lehr via llvm-commits]

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

Thank you for working through the issues the bots reported. Feel free to land when convenient for you.

https://github.com/llvm/llvm-project/pull/154211