[AMDGPU] NFC. Assert that mask is full with VOPC DPP

VOPC DPP should not be formed when the row_mask and bank_mask are not
0xf (full) because the resulting VOP DPP would have different semantics
than the MOV DPP followed by VOP. Existing checks in GCNDPPCombine cover
this case but for different reasons, so assert the property for
future-proofing.

Reviewed By: nhaehnle

Differential Revision: https://reviews.llvm.org/D130101