[X86] Add ISel patterns to select SSE3/AVX ADDSUB instructions.

This patch adds ISel patterns to select SSE3/AVX ADDSUB instructions
from a sequence of "vadd + vsub + blend".

Example:

///
typedef float float4 __attribute__((ext_vector_type(4)));

float4 foo(float4 A, float4 B) {
  float4 X = A - B;
  float4 Y = A + B;
  return (float4){X[0], Y[1], X[2], Y[3]};
}
///

Before this patch, (with flag -mcpu=corei7) llc produced the following
assembly sequence:
  movaps  %xmm0, %xmm2
  addps   %xmm1, %xmm2
  subps   %xmm1, %xmm0
  blendps $10, %xmm2, %xmm0


With this patch, we now get a single
  addsubps  %xmm1, %xmm0