Simplify code by converting multiple places that were manually concatenating 128...

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@155318 91177308-0d34-0410-b5e6-96231b3b80d8