[SDAG] Fix pow2 assumption when splitting vectors

When reducing vector builds to shuffles it possible that
the DAG combiner may try to extract invalid subvectors.

This happens as the existing code assumes vectors will be power
of 2 sizes, which is already untrue, but becomes more noticable
with v6 and v7 types.
Specifically the existing code assumes that half PowerOf2Ceil of
a given vector index will fit twice into a given vector.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D103880