[FPEnv][InstCombine] Correct strictfp tests.

Correct InstCombine 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. After D154991 the constrained intrinsics have the
strictfp attribute by default so they don't need it here, but other
functions do.

Test changes verified with D146845.