[DAGCombiner] Expand combining of FP logical ops to sign-setting FP ops

We already do the following combines:
(bitcast int (and (bitcast fp X to int), 0x7fff...) to fp) -> fabs X
(bitcast int (xor (bitcast fp X to int), 0x8000...) to fp) -> fneg X

When the target has "bit preserving fp logic". This patch just extends it
to also combine:
(bitcast int (or (bitcast fp X to int), 0x8000...) to fp) -> fneg (fabs X)

As some targets have fnabs and even those that don't can efficiently lower
both the fabs and the fneg.

Differential revision: https://reviews.llvm.org/D44548

llvm-svn: 344093