Simplify code by converting multiple places that were manually concatenating...

Simplify code by converting multiple places that were manually concatenating 128-bit vectors to use either CONCAT_VECTORS or a helper function. CONCAT_VECTORS will itself be lowered to the same pattern as before. The helper function is needed for concats of BUILD_VECTORs since getNode(CONCAT_VECTORS) will just return a large BUILD_VECTOR and we may be trying to lower large BUILD_VECTORS when this occurs.

llvm-svn: 155318