[X86] Teach FCOPYSIGN lowering to recognize constant magnitudes.

For code like:
    float foo(float x) { return copysign(1.0, x); }
We used to generate:
    andps  <-0.000000e+00,0,0,0>, %xmm0
    movss  <1.000000e+00>, %xmm1
    andps  <nan>, %xmm1
    orps   %xmm0, %xmm1
Basically doing an abs(1.0f) in the two middle instructions.

We now generate:
    andps  <-0.000000e+00,0,0,0>, %xmm0
    orps   <1.000000e+00,0,0,0>, %xmm0

Builds on cleanups r223415, r223542.
rdar://19049548
Differential Revision: http://reviews.llvm.org/D6555

llvm-svn: 225357