spirv: Lower OpConstantNull to proper values
This is an alternative for !769 (closed). There's a lot of preparing ground so we can use the address format to query its expected null pointer value and use that as the constant (instead of using just zeros). For some address formats in use, 0 is not the right null pointer value.
Note: this is groundwork to get OpPtrEqual and OpPtrNotEqual from SPIR-V 1.4.