[llvm] [AMDGPU][True16][CodeGen] replace subreg_to_reg to req_sequence (PR #138746)

[Joe Nash via llvm-commits]

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

LGTM as well. I honestly would have expected it to be NFC. But the changes look to be minor.

https://github.com/llvm/llvm-project/pull/138746