[PowerPC] Add a new test for vperm with a swapped vector operand and a constant pool

This patch adds a new test that includes a vperm instruction with xxswapd as its
vector operand on little-endian Power8. The test demonstrates the constant pool
for the mask operand, which is intended to indicate the optimization of vperm
and the modification of the constant pool in subsequent patches.

Reviewed By: amyk

Differential Revision: https://reviews.llvm.org/D148942