[LegalizeTypes] Expand FNEG to bitwise op for IEEE FP types

Summary:
Except for custom floating point types x86_fp80 and ppc_fp128,
expand Y = FNEG(X) to Y = X ^ sign mask to avoid library call.
 Using bitwise operation can improve code size and performance.

Reviewers: efriedma

Reviewed By: efriedma

Subscribers: efriedma, kpn, arsenm, eli.friedman, javed.absar, rbar, johnrusso, simoncook, sabuasal, niosHD, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, PkmX, jocewei, asb, llvm-commits

Differential Revision: https://reviews.llvm.org/D57875

llvm-svn: 353757