[GISel] Legalize G_FSUB to G_FADD + G_FNEG even if G_FNEG is illegal

`G_FNEG` used to be legalized to `G_FSUB -0, x` causing infinite loop.
This is no longer the case after D84287.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D148187