transform obscured FP sign bit ops into a fabs/fneg using TLI hook
This is effectively a revert of:
http://reviews.llvm.org/rL249702 - [InstCombine] transform masking off of an FP sign bit into a fabs() intrinsic call (PR24886)
http://reviews.llvm.org/rL249701 - [ValueTracking] teach computeKnownBits that a fabs() clears sign bits
and a reimplementation as a DAG combine for targets that have IEEE754-compliant fabs/fneg instructions.
This is intended to resolve the objections raised on the dev list:
In the interest of patch minimalism, I've only partly enabled AArch64. PowerPC, MIPS, x86 and others can enable later.
Differential Revision: http://reviews.llvm.org/D19391