Fix (bitcast (fabs x)), (bitcast (fneg x)) and (bitcast (fcopysign cst,

x)) combines for ppc_fp128, since signbit computation is more
complicated.

Discussion thread:
http://lists.llvm.org/pipermail/llvm-dev/2015-November/092863.html

Patch by Tim Shen!

llvm-svn: 255305