Merge expression simplifier and dead branch remover into the pure optimizer

[kdy1]

Currently, those passes only use the main thread, but those passes are mostly stateless and we need to move logic to minifier anyway, because of side effect check.

I profiled it, and those two passes used 7.5% of main thread execution time

cc @Austaras

[kdy1]

I think this can be done by exposing shallow optimizers from swc_ecma_transforms_optimization, but with a comment that says it's not considered as a public API.

@Austaras How do you think about this?

[Austaras]

What shallow optimizer?

[kdy1]

Code except visit_mut_children_with(self), e.g.

https://github.com/swc-project/swc/blob/63efd0089b1249b4f728c0c3a16fb2a1e97747be/crates/swc_ecma_transforms_optimization/src/simplify/expr/mod.rs#L1165-L1274

[Austaras]

That seems to be good enough for expr_simplier

[Austaras]

What would be the potential performance impact of check is global var? It seems like need ctxt.outer() which need to acquire global lock

[kdy1]

We can pre-compute SyntaxContext and compare it with Ident.span.ctxt

[Austaras]

Come to revisit this, I believe proper solution for expr_simplifier would be separate it into two parts -- AST mangle and evaluate. Former part should be run only once before all pass, and later one should be merged with pure. I don't know if that would be a break change for the JS side.

[kdy1]

I think it's good way to go and I don't think it's a breaking change for js side