[FPEnv] IRBuilder fails to add strictfp attribute

The strictfp attribute is required on all function calls in a function
that is itself marked with the strictfp attribute. The IRBuilder knows
this and has a method for adding the attribute to function call instructions.

If a function being called has the strictfp attribute itself then the
IRBuilder will refuse to add the attribute to the calling instruction
despite being asked to add it. Eliminate this error.

Differential Revision: https://reviews.llvm.org/D84878