[OpenMP] Adjust phases for AMDGPU offloading for OpenMP in save-temps mode

Currently, the behaviour of `-save-temps` changes the generated output
when offloading to AMDGPU. This is because we only have a single phase
and it contains the `-disable-llvm-passes` flags which results in
unoptimized bitcode. We need to make sure we generate another phase that
produces both the optimized and unoptimized bitcode. There used to be a
check that turned these phases into a no-op. But I believe it is more
correct to not generate them this way in the first place. Doing this
requires a bit of a hack, replacing an already generated phase action,
but it should be fine.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D141440