[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

llvm-svn: 298939

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -971,6 +971,8 @@ static FastMathFlags getDecodedFastMathFlags(unsigned Val) {
     FMF.setNoSignedZeros();
   if (0 != (Val & FastMathFlags::AllowReciprocal))
     FMF.setAllowReciprocal();
+  if (0 != (Val & FastMathFlags::AllowContract))
+    FMF.setAllowContract(true);
   return FMF;
 }