Fix #997: Limited support for in-pattern regex flags.

After the `rhs` is constructed in `x1`:
```javascript
  if ((x1 !== null)) {
    var jspattern = ScalaJS.as.T(x1.$$und1$f);
    var flags1$1 = ScalaJS.uI(x1.$$und2$f);
    var jsx$1 = new ScalaJS.c.T2().init___O__O(jspattern, flags1$1)
  } else {
    var jsx$1;
    throw new ScalaJS.c.s_MatchError().init___O(x1)
  };
  this.x$1$1 = jsx$1;
  this.jspattern$1 = ScalaJS.as.T(this.x$1$1.$$und1$f);
  var this$17 = this.x$1$1;
  this.flags1$1 = ScalaJS.uI(this$17.$$und2$f);
```