[GlobalISel] Add default action for G_FNEG

Summary: rL297171 introduced G_FNEG for floating-point negation instruction and IRTranslator started to translate `FSUB -0.0, X` to `FNEG X`. This patch adds a default action for G_FNEG to avoid breaking existing targets.

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

Reviewed By: qcolombet

Subscribers: dberris, rovka, llvm-commits

Differential Revision: https://reviews.llvm.org/D30721

llvm-svn: 297301