[llvm] [AMDGPU] Reimplement V_READFIRSTLANE_B32 as a normal VOP1 Pseudo. NFCI. (PR #81877)

[Jay Foad via llvm-commits]

jayfoad wrote:

This was a weird case that didn't have the usual Pseudo/Real abstraction.

https://github.com/llvm/llvm-project/pull/81877