fix(ecmascript): eval assignop to the ident (#4609)
### Description
- closes WEB-889
When there is a variable with `+=` assignop only to the given ident
```
var css = '';
for (var i = 0; i < length; i++) {
var partRule = this.rules[i];
if (typeof partRule === 'string') {
css += partRule;
} else if (partRule) {
css += partString;
}
}
if (css) {
....
}
```
evaluated result becomes
```
var css = '';
for(var i = 0; i < length; i++){
var partRule = this.rules[i];
if (typeof partRule === 'string') {
css += partRule;
} else if (partRule) {
css += partString;
}
}
if ("TURBOPACK compile-time falsy", 0) {
"TURBOPACK unreachable";
}
```
so `if (css)` condition never executed even though it can be executed
depends on the runtime assignop condition.
PR attempts to evaluate if assignop is for the ident to prevent this.
---------
Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>
