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.
When we see a switch block which only contains one default block, emit a
do {} while(false) statement instead, which is far more idiomatic and
readable anyways.
Inner scope can still dominate here, so we need to be conservative when
we observe switch blocks specifically. Normal selection merges cannot
merge from multiple paths.
