Don't pass FPOpFusion::Strict to the backend

This restores the behavior prior to D31167 where the code-gen default was
FPC_On which mapped to FPOpFusion::Standard.  After merging the FE
state (on/off) and the code-gen state (on/fast/off), the default became off to
match the front-end.

In other words, the front-end controls when to fuse along the language
standards and the backend shouldn't override this by splitting fused
intrinsics as FPOpFusion::Strict would imply.

Differential Revision: https://reviews.llvm.org/D32301

llvm-svn: 300858