[PPC] Try to fix builbots

Attempt to handle unsupported types, such as structs, in
getMemoryOpCost. The backend now checks for a supported type and
calls into BasicTTI as a fallback. BasicTTI will now also perform
the same check and will default to an expensive cost of 4 for 'Other'
MVTs.

Differential Revision: https://reviews.llvm.org/D80984