Don't assert in BasicTTI::getMemoryOpCost for non-simple types

BasicTTI::getMemoryOpCost must explicitly check for non-simple types; setting
AllowUnknown=true with TLI->getSimpleValueType is not sufficient because, for
example, non-power-of-two vector types return non-simple EVTs (not MVT::Other).

llvm-svn: 206150