AMDGPU/GlobalISel: Select G_FABS/G_FNEG

f64 doesn't work yet because tablegen currently doesn't handlde
REG_SEQUENCE.

This does regress some multi use VALU fneg cases since now the
immediate remains in an SGPR, and more moves are used for legalizing
the xor. This is a SIFixSGPRCopies deficiency.

llvm-svn: 371540