[X86] Simplify some code in lowerV4X128VectorShuffle and lowerV2X128VectorShuffle

Previously we extracted two subvectors and concatenate. But the concatenate will be lowered to two insert subvectors. Then DAG combine will merge once of the inserts and one of the extracts back into the original vector. We might as well just directly use one extract and one insert.

llvm-svn: 324710