[DAGCombiner] Merge SIGN_EXTEND_INREG vector constant folding methods. NCI.

visitSIGN_EXTEND_INREG calls SelectionDAG::getNode to constant fold scalar constants but handles vector constants itself, despite getNode being capable of dealing with them.

This required a minor change to the getNode implementation to actually deal with cases where the scalars of a BUILD_VECTOR were wider integers than the vector type - which was the only extra ability of the visitSIGN_EXTEND_INREG implementation.

No codegen intended and all existing tests remain the same.

llvm-svn: 249236