[InstCombine] fold shuffle of fabs

shuffle (fabs X), Mask --> fabs (shuffle X, Mask)
shuffle (fabs X), (fabs Y), Mask --> fabs (shuf X, Y, Mask)

https://alive2.llvm.org/ce/z/JH2nkf

This generalizes the existing fneg transforms to also work with fabs.

A likely follow-up would generalize this further to move any unary
intrinsic op.