[llvm] [AMDGPU][True16][CodeGen] update zext pattern with reg_sequence (PR #154952)

[Matt Arsenault via llvm-commits]

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

The high half should probably be using v_mov_b32 instead of s_mov_b32 but that's a separate change 

https://github.com/llvm/llvm-project/pull/154952