[X86] combineFneg - generalize FMA negations with isNegatibleForFree/getNegatedExpression

[X86] combineFneg - generalize FMA negations with isNegatibleForFree/getNegatedExpression

This has a really interesting side effect in that it improves some UMAX/UMIN reduction code which had redundant XOR(SHUFFLE(XOR(X,SIGNMASK)),SIGNMASK) patterns - the getNegatibleCost recognises it as FNEG(SHUFFLE(FNEG(X))).... We have a lot of FNEG patterns bitcasted to the integer domain for XOR signbit twiddling which is similar to what we do to allow UMAX/UMIN to be lowered using SMAX/SMIN.

Differential Revision: https://reviews.llvm.org/D74231