Revert "Revert "[FPEnv] Teach the IRBuilder about invoke's correct use of the strictfp attribute.""

Similar to D69312, and documented in D69839, the IRBuilder needs to add
the strictfp attribute to invoke instructions when constrained floating
point is enabled.

This is try 2, with the test corrected.

Differential Revision: https://reviews.llvm.org/D93134