[GlobalISel] LegalizerHelper: Lower (G_FSUB X, Y) to (G_FADD X, (G_FNEG Y))

Summary: No test case as none of the in-tree targets with GlobalISel support has this condition.

Reviewers: qcolombet, aditya_nandakumar, dsanders, t.p.northover, ab

Reviewed By: qcolombet

Subscribers: dberris, rovka, kristof.beyls, llvm-commits, igorb

Differential Revision: https://reviews.llvm.org/D30786

llvm-svn: 297512