[PATCH] [X86] Teach FCOPYSIGN lowering to recognize constant magnitudes.

[Ahmed Bougacha]

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

http://reviews.llvm.org/D6555

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/copysign-constant-magnitude.ll
  test/CodeGen/X86/copysign-zero.ll
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