Add the PPC fcpsgn instruction

Modern PPC cores support a floating-point copysign instruction, and we can use
this to lower the FCOPYSIGN node (which is created from calls to the libm
copysign function). A couple of extra patterns are necessary because the
operand types of FCOPYSIGN need not agree.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188653 91177308-0d34-0410-b5e6-96231b3b80d8