[llvm] [AMDGPU] Allow hoisting of V_READFIRSTLANE_B32 for uniform operand (PR #178312)

[Stanislav Mekhanoshin via llvm-commits]

rampitec wrote:

On a separate but related note: I wrote and deleted code to implement `ISD::AssertUniform` 3 times already.

https://github.com/llvm/llvm-project/pull/178312