Lower BUILD_VECTOR to SHUFFLE + INSERT_VECTOR_ELT for X86

- If INSERT_VECTOR_ELT is supported (above SSE2, either by custom
  sequence of legal insn), transform BUILD_VECTOR into SHUFFLE +
  INSERT_VECTOR_ELT if most of elements could be built from SHUFFLE with few
  (so far 1) elements being inserted.

llvm-svn: 166288