bug: compound function call unsafe to combine

[unbyte]

see: mishoo/UglifyJS#1631

TL;DR

before

{
    let pc = 0;
    function f(x) {
        pc = 200;
        return 100;
    }
    function x() {
        const t = f();
        pc += t;
        return pc;
    }
    console.log(x());
}

// output: 300

after

{let t=function(c){return n=200,100},o=function(){return n+=t(),n};r=t,e=o;let n=0;console.log(o())}

// output: 100

The reason why this bug was not found before is that the test case uses var instead of let/const, and esbuild skips inlining those variables when meets var.

[unbyte]

I'm trying enabling the optimization for vars, so I may have the chance to try to fix this bug.