[IR] Add AllowContract to FastMathFlags

[IR] Add AllowContract to FastMathFlags

-ffp-contract=fast does not currently work with LTO because it's passed as a
TargetOption to the backend rather than in the IR. This adds it to
FastMathFlags.

This is toward fixing PR25721

Differential Revision: https://reviews.llvm.org/D31164

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298939 91177308-0d34-0410-b5e6-96231b3b80d8