Dead code elimination can eliminate side-effecting code

[bakkot]

Copying from https://github.com/babel/minify/issues/930#issuecomment-439339599 to make sure it gets tracked:

'use strict';
function foo() {
    if (bar(), true) {
        baz();
    } else {
        NOT_REACHABLE();
    }
}

with dead code and simplify gets transformed to

"use strict";function foo(){baz()}

which is wrong - the call to bar() can have side effects.

I expect the bug is here: this code assumes that anything which .evaluate()s confidently can be safely replaced with its evaluation, but that's not true - you can have side effects even for things you're confident about. Compare the more complex code in the path for IfStatement, which was added in #386 to fix #385. I imagine the fix is basically copying that code to the ConditionalExpression case; probably a good first issue for someone.

(This issue is pretty much the same as https://github.com/babel/minify/issues/385#issuecomment-274338700, just with slightly different input conditions.)