[llvm] [AMDGPU] Add readfirstlane for inline asm SGPR with VGPR input (PR #176330)

[Matt Arsenault via llvm-commits]

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arsenm wrote:

You have all of the set code to avoid repeatedly processing the same values, but none of these tests use the same register or value repeatedly 

https://github.com/llvm/llvm-project/pull/176330