[llvm] [AMDGPU] Fix combineMasks dropping condition (PR #203180)

[Arseniy Obolenskiy via llvm-commits]

aobolensk wrote:

> I think `S_AND exec, (S_AND x, x)` should work as expected, no?

No, that's exactly what is being fixed here. When the nested S_AND x, x is the first operand before this patch exec was picked as the UniqueOpnd and it produced producing S_AND exec, exec

https://github.com/llvm/llvm-project/pull/203180