fix unnecessary parentheses for binary operators and add regression test

Author: michaelficarra

escodegen.js

@@ -1835,7 +1835,7 @@
         BinaryExpression: function (expr, precedence, flags) {
             var result, leftPrecedence, rightPrecedence, currentPrecedence, fragment, leftSource;
             currentPrecedence = BinaryPrecedence[expr.operator];
-            leftPrecedence = expr.operator === '**' ? Precedence.Postfix : currentPrecedence + 1;
+            leftPrecedence = expr.operator === '**' ? Precedence.Postfix : currentPrecedence;
             rightPrecedence = expr.operator === '**' ? currentPrecedence : currentPrecedence + 1;
 
             if (currentPrecedence < precedence) {

test/compare/binary-precedence.expected.js

@@ -0,0 +1,4 @@
+a + b + c;
+a + (b + c);
+a + b * c;
+a * b + c;

test/compare/binary-precedence.js

@@ -0,0 +1,4 @@
+(a + b) + c;
+a + (b + c);
+a + b * c;
+a * b + c;