Inliner incorrectly modifies IINC on parameters

[SolalPirelli]

scala/scala@bfa1380#diff-6402881c68d306988091172b129869daad7519257b48d97d2632465789d7383a added handling for VarInsnNode at (new) line 744 of Inliner.scala but missed changing the IincInsnNode case just below.

Thus, IINC instructions aren't properly modified when their target is a parameter.

This was found while porting the optimizer to Scala 3, in which the following is a repro:

//> using options -opt-inline:**

object Test {
  def foo(b: Option[Int]): Unit =
    fooRec(x = 0, b)

  private def fooRec(x: Int, b: Option[Int]): Unit = {
    if (b.nonEmpty) {
      fooRec(x + 1, b)
    }
  }

  def main(args: Array[String]): Unit = {
    foo(None)
  }
}

I haven't actually tested it with scala2 but the underlying problem should also exist.