Ensure that the backend no longer emits unnecessary vector insert instructions

immediately after SSE scalar fp instructions like addss or mulss.

Added patterns to select SSE scalar fp arithmetic instructions from a scalar
fp operation followed by a blend.

For example, given the following code:
  __m128 foo(__m128 A, __m128 B) {
    A[0] += B[0];
    return A;
  }

previously we generated:
  addss %xmm0, %xmm1
  movss %xmm1, %xmm0

now we generate:
  addss %xmm1, %xmm0

llvm-svn: 196925