[FPEnv][AMDGPU] Correct strictfp tests.

Correct AMDGPU strictfp tests to follow the rules documented in the LangRef:
https://llvm.org/docs/LangRef.html#constrained-floating-point-intrinsics

Mostly these tests just needed the strictfp attribute on function
definitions.  I've also removed the strictfp attribute from uses
of the constrained intrinsics because it comes by default since
D154991, but I only did this in tests I was changing anyway.

I also removed attributes added to declare lines of intrinsics. The
attributes of intrinsics cannot be changed in a test so I eliminated
attempts to do so.

Test changes verified with D146845.