[AArch64 NEON] Fix a bug caused by undef lane when generating VEXT.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199628 91177308-0d34-0410-b5e6-96231b3b80d8
Fix all the remaining lost-fast-math-flags bugs I've been able to find.  The most important of these are cases in the generic logic for combining BinaryOperators.
This logic hadn't been updated to handle FastMathFlags, and it took me a while to detect it because it doesn't show up in a simple search for CreateFAdd.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199629 91177308-0d34-0410-b5e6-96231b3b80d8