Ensure that SIMD fields are correctly typed (#24377)

Ensure that SIMD fields are correctly typed (#24377)

When a struct field is imported, its type needs to be normalized.
Also, the LHS of a struct init, even if a SIMD type, should not be transformed to a non-block node,
except in the case of a SIMD local, in which case it must be transformed to a simple assignment.
Also, add an assert to catch this kind of bug in liveness.

Fix #24336