fix: expand array synonyms / subset types in LHS assignments

[RustanLeino]

Previously, the type of LHSs in simultaneous assignments was used without first calling NormalizeExpand(). This had caused a crash when a LHS was a type synonym or subset type around an array type.

Fixes #2883

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[MikaelMayer]

Good job fixing this crash. I'm not convinced however based on the code that TypeArgs is always non-empty at this point, but I guess I don't have the entire context and assumptions here.

[RustanLeino]

Since this code is in the verifier, we know the program has passed the resolver. The only legal types (after expanding synonyms and subset types) for SeqSelectExpr and MultiSelectExpr are seq and arrayN types, and each of these will have the TypeArgs field filled in (with at least one element).