[FPEnv][AMDGPU] Correct AMDGPUSimplifyLibCalls handling of strictfp attribute. (#86705)

The AMDGPUSimplifyLibCalls pass was lowering function calls with the
strictfp attribute to sequences that included function calls incorrectly
lacking the attribute. This patch corrects that.

The pass now also emits the correct constrained fp call instead of
normal FP instructions when in a function with the strictfp attribute.
Replacing non-constrained calls with constrained calls when required
is still on the IRBuilder's TODO list.