[ARM GlobalISel] Legalize G_FNEG for s32 and s64

For hard float, it is legal.

For soft float, we need to lower to 0 - x first, and then we can use the
libcall for G_FSUB. This is undoing some of the canonicalization
performed by the IRTranslator (which introduces G_FNEG when it sees a
0 - x). Ideally, that canonicalization would be performed by a
pre-legalizer pass that would allow targets to opt out of this behaviour
rather than dance around it in the legalizer.

llvm-svn: 322168