Fixed DAGCombiner bug (found and localized by James Malloy):

The DAGCombiner tries to optimise a BUILD_VECTOR by checking if it
consists purely of get_vector_elts from one or two source vectors. If
so, it either makes a concat_vectors node or a shufflevector node.

However, it doesn't check the element type width of the underlying
vector, so if you have this sequence:

Node0: v4i16 = ...
Node1: i32 = extract_vector_elt Node0
Node2: i32 = extract_vector_elt Node0
Node3: v16i8 = BUILD_VECTOR Node1, Node2, ...

It will attempt to:

Node0:    v4i16 = ...
NewNode1: v16i8 = concat_vectors Node0, ...

Where this is actually invalid because the element width is completely
different. This causes an assertion failure on DAG legalization stage.

Fix:
If output item type of BUILD_VECTOR differs from input item type.
Make concat_vectors based on input element type and then bitcast it to the output vector type. So the case described above will transformed to:
Node0:    v4i16 = ...
NewNode1: v8i16 = concat_vectors Node0, ...
NewNode2: v16i8 = bitcast NewNode1

llvm-svn: 162195