[DAG] Allow build-to-shuffle combine to combine builds from two wide vectors.

This allows us to, in some cases, create a vector_shuffle out of a build_vector, when
the inputs to the build are extract_elements from two different vectors, at least one
of which is wider than the output. (E.g. a <8 x i16> being constructed out of
elements from a <16 x i16> and a <8 x i16>).

Differential Revision: https://reviews.llvm.org/D24491

llvm-svn: 281402