[PPC] Give unaligned memory access lower cost on processor that supports it

Newer ppc supports unaligned memory access, it reduces the cost of unaligned memory access significantly. This patch handles this case in PPCTTIImpl::getMemoryOpCost.

This patch fixes pr31492.

Differential Revision: https://reviews.llvm.org/D28630

This is resubmit of r292680, which was reverted by r293092. The internal application failures were actually caused by a source code bug.

llvm-svn: 295506