Preserve byte_extract structure in trace RHS for incremental SMT2 backend

[tautschnig]

When ssa_full_lhs is a byte_extract expression, resolve operands individually instead of simplifying the entire expression. This prevents the simplifier from evaluating away the byte_extract structure when the SMT2 backend returns a dense array_exprt.

Also remove the no-new-smt tag from the unbounded_array regression test since it now works with the incremental SMT2 backend.

Fixes: #8076

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