[PowerPC][AIX] Allow VSX patterns to be 32-bit and 64-bit safe on P8+.

This patch updates two patterns involving `scalar_to_vector` and
`SCALAR_TO_VECTOR_PERMUTED` nodes to be safe for both 64-bit and 32-bit by
pulling the patterns out of the 64-bit specific guard. These patterns are
matched on POWER8 and above.

Differential Revision: https://reviews.llvm.org/D125389