[DAGCombiner] Add a DAG combine to turn a splat build_vector where the splat...

[DAGCombiner] Add a DAG combine to turn a splat build_vector where the splat elemnt is a bitcast from a vector type into a concat_vector

For example, a build_vector of i64 bitcasted from v2i32 can be turned into a concat_vectors of the v2i32 vectors with a bitcast to a vXi64 type

Differential Revision: https://reviews.llvm.org/D42090

llvm-svn: 322811