[SystemZ] Avoid type legalization on structs

In SystemZTTIImpl::getMemoryOpCost, the call to getNumberOfParts will
run type legalization, which can't handle structs. So before that, we
check for an unknown value type and forward to BaseT, just like many
other targets do in this situation.

https://bugzilla.redhat.com/show_bug.cgi?id=2224885

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D156379