Expand fuzz_simplify to test more expressions

We aren't catching enough bugs in `test/correctness/fuzz_simplify.cpp` because the set of expressions it tries to generate is incomplete.

For instance, see Issue #9030 -- this bug was discovered only via an additional `extract_lanes` fuzz tester that is present only in PR #8629. But really, this functionality should have been in the existing simplifier all along.