Improve the pattern match compilation with more annotations

[bobzhang]

type t = 
  | A
  | B 
  | C 
  | D of int 
  | E of int


let f x = match x with 
  | A -> 0 
  | B -> 1 
  | C -> 2 
  | D x -> x 
  | E x -> x + 1

Current output:

function f(x) {
  if (typeof x !== "number") {
    if (x.TAG) {
      return x._0 + 1 | 0;
    } else {
      return x._0;
    }
  }
  switch (x) {
    case /* A */0 :
        return 0;
    case /* B */1 :
        return 1;
    case /* C */2 :
        return 2;
    
  }
}

Expected output:

function f(x) {
  if (typeof x !== "number") {
    if (x.TAG === 0 /* E */ ) {
      return x._0 + 1 | 0;
    } else {
      return x._0;
    }
  }
  switch (x) {
    case /* A */0 :
        return 0;
    case /* B */1 :
        return 1;
    case /* C */2 :
        return 2;
    
  }
}