[llvm] [AMDGPU][ISel] Reduce `f64` compare to integer compare of upper half (PR #188356)

[via llvm-commits]

zGoldthorpe wrote:

I added a test to llvm/llvm-test-suite#378, though it's more to get that conversation moving, as I'm not very familiar with that repo.

By the way, assuming this PR is still good after the recent change, is it fine to merge while execution tests are in development?

https://github.com/llvm/llvm-project/pull/188356