[llvm] [AMDGPU] NFC, fix geps, add vector-combine pass to promote-alloca-array-to-vector test (PR #166280)

[Shilei Tian via llvm-commits]

shiltian wrote:

Right, so you can just run the promote pass, take the output of it, and then add a separate test case for vector combine.

https://github.com/llvm/llvm-project/pull/166280