Teach GetNegatedExpression to negate 0-B to B in UnsafeFPMath mode, and

visitFSUB to fold 0-B to -B in UnsafeFPMath mode. Also change visitFNEG
to use isNegatibleForFree/GetNegatedExpression instead of doing a subset
of the same thing manually.

This fixes test/CodeGen/X86/negative-sin.ll.

llvm-svn: 37842