[SelectionDAG][PowerPC] Memset reuse vector element for tail store

On PPC there are instructions to store element from vector(e.g.
stxsdx/stxsiwx), and these instructions can be leveraged to avoid tail
constant in memset and constant splat array initialization.

This patch tries to explore these opportunities.

Reviewed By: shchenz

Differential Revision: https://reviews.llvm.org/D138883