aco: fallback if instr_info.definition_size is zero
For non-arithmetic opcodes such as buffer_load_dword, aco_opcodes.py can't determine the definition size. Just use the definition size instead.
This fixes a failure with some register allocations of dEQP-VK.spirv_assembly.instruction.graphics.16bit_storage.uniform_32struct_to_16struct.uniform_buffer_block_geom
(before !7161 (merged) fixed loop unrolling)