[llvm] [AMDGPU] Properly check op_sel in GCNDPPCombine (PR #79122)

[Mariusz Sikora via llvm-commits]

mariusz-sikora-at-amd wrote:

> Overall looks great, thanks for fixing it so fast. I think it needs a test to reject formation when DST_OP_SEL is 1, but otherwise LGTM.
I verified the change, we will have tests in these PR https://github.com/llvm/llvm-project/pull/78414
https://github.com/llvm/llvm-project/pull/78414/files#diff-4f33ce09676ec00dd307fa51d99e0277c4e65d89d8996fc9ed419eabbd17fe1fR7


https://github.com/llvm/llvm-project/pull/79122