[AMDGPU] Fix handling of void types in isLegalAddressingMode

It is legal for the type passed to isLegalAddressingMode to be
unsized or, more specifically, VoidTy. In this case, we must
check the legality of load / stores for all legal types. Directly
trying to call getTypeStoreSize is incorrect, and leads to breakage
in e.g. Loop Strength Reduction. This change guards against that
behaviour.

Differential Revision: https://reviews.llvm.org/D40405

llvm-svn: 332409