[llvm] [SPIR-V] Add lowering for G_FPOWI (PR #185454)

[Diego Novillo via llvm-commits]

dnovillo wrote:

> LGTM, My only issue is a small thing about the tests. I want to validate the spir-v generated for vulkan in the vulkan env. It makes the testing a little harder because we have to avoid the linkage attribute.

Thanks. I've updated the PR with these changes. PTAL.

https://github.com/llvm/llvm-project/pull/185454