Fix #14773: Reuse the param slots for the tailrec local mutable vars.

[sjrd]

The tailrec phase generates code that looks like

  class Bar {
    def foo(x1: Int, x2: Int): Int = {
      var this$: Bar = this;
      var taillocal$x1: Int = x1;
      var taillocal$x2: Int = x2;
      ...
      // body where `this`, `x1` and `x2` are replaced by
      // `this$`, `taillocal$x1` and `taillocal$x2`
    }
  }

This generates bytecode where the this value and the parameters never actually change, and are never used. Instead, the synthetic mutable variables are used instead.

As described in the linked issue, this confuses debuggers, which only display the never-changing original this value and parameters.

In this commit, we intercept this shape of code in the back-end. We reliable identify tailrec-generated ValDefs from their semantic names, with an additional safety check that they are Synthetic | Mutable. When we find this shape, we do not allocate local slots for the vars, and instead reuse the slots for this and the parameters. We skip past the ValDefs so that code generation does not re-emit useless assignments.

[smarter]

@lrytz do you want to review this one?

[lrytz]

👍 LGTM!