[InstCombine] Complete folding of fneg-of-fabs

This patch add a function foldSelectWithFCmpToFabs, and do more combine for
fneg-of-fabs.
With 'nsz':
fold (X <  +/-0.0) ? X : -X or (X <= +/-0.0) ? X : -X to -fabs(x)
fold (X >  +/-0.0) ? X : -X or (X >= +/-0.0) ? X : -X to -fabs(x)

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D123830