Unnecessary calls to `TypeChecker.checkCast` for compile time know subtypes

[manuranga]

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Consider the following code.

const OFF = 0;
type Color "red" | "green" | "blue" | OFF;

function parseColor(string c) returns Color {
   if c is "r" {
       return "red";
   }
   else if c is "g" {
       return "green";
   }
   else if c is "b" {
       return "blue";
   }
   else if c is "0" {
       return OFF;
   }
   panic error("unexpected input : " + c);
}

In this there is no need to check each given constant (eg "red") is a subtype of Color since it's known at compile time.