[PowerPC] Reduce symmetrical swaps for lane-insensitive vector ops

This patch simplifies pattern (xxswap (vec-op (xxswap a) (xxswap b)))
into (vec-op a b) if vec-op is lane-insensitive. The motivating case
is ScalarToVector-VecOp-ExtractElement sequence on LE, but the
peephole itself is not related to endianness, so BE may also benefit
from this.

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D97658