[SelectionDAG] Fix problematic call to EVT::changeVectorElementType().

The function changeVectorElementType assumes MVT input types will
result in MVT output types.  There's no gurantee this is possible
during early code generation and so this patch converts an instance
used during initial DAG construction to instead explicitly create a
new EVT.

NOTE: I could have added more MVTs, but that seemed unscalable as
you can either have MVTs with 100% element count coverage or 100%
bitwidth coverage, but not both.

Differential Revision: https://reviews.llvm.org/D157392