Revert "js: Remove type from VariableInput"

This reverts commit 8f66924fe9.

src/compiler/jsgen.js

@@ -158,6 +158,7 @@ class SafeConstantInput extends ConstantInput {
 class VariableInput {
     constructor (source) {
         this.source = source;
+        this.type = TYPE_UNKNOWN;
         /**
          * The value this variable was most recently set to, if any.
          * @type {Input}
@@ -176,17 +177,26 @@ class VariableInput {
             return;
         }
         this._lastInput = input;
+        if (input instanceof TypedInput) {
+            this.type = input.type;
+        } else {
+            this.type = TYPE_UNKNOWN;
+        }
     }
 
     asNumber () {
+        if (this.type === TYPE_NUMBER) return this.source;
+        if (this.type === TYPE_NUMBER_NAN) return `(${this.source} || 0)`;
         return `(+${this.source} || 0)`;
     }
 
     asString () {
+        if (this.type === TYPE_STRING) return this.source;
         return `("" + ${this.source})`;
     }
 
     asBoolean () {
+        if (this.type === TYPE_BOOLEAN) return this.source;
         return `toBoolean(${this.source})`;
     }