Undef values may be of struct type and may be used in constants.
Therefore, they must be interleaved with constants and types.
Fixes the rest of the Vulkan CTS test
`dEQP-VK.spirv_assembly.instruction.compute.opundef.undefined_spec_constant_composite`.
(Please excuse the churn in the reference output; it's an inevitable
result of this change.)
Speculate that we can modify the SSA value in-place. As long as it is
not used after the modify, this is fine.
Also need to make sure we don't attempt to RMW something that is
impossible to modify.