Correct parentheses for mixed exp/mod (#5243)

Fixes #5238.

cc @duailibe for the [original change](https://github.com/prettier/prettier/pull/4413). I basically undid that PR because it didn't look to me like `shouldFlatten` made sense as the place to introduce the behavior it was going for, but I might have misunderstood something.

src/common/util.js

@@ -267,10 +267,6 @@ const equalityOperators = {
   "===": true,
   "!==": true
 };
-const additiveOperators = {
-  "+": true,
-  "-": true
-};
 const multiplicativeOperators = {
   "*": true,
   "/": true,
@@ -284,11 +280,6 @@ const bitshiftOperators = {
 
 function shouldFlatten(parentOp, nodeOp) {
   if (getPrecedence(nodeOp) !== getPrecedence(parentOp)) {
-    // x + y % z --> (x + y) % z
-    if (nodeOp === "%" && !additiveOperators[parentOp]) {
-      return true;
-    }
-
     return false;
   }
 

src/language-js/needs-parens.js

@@ -291,10 +291,10 @@ function needsParens(path, options) {
           }
 
           if (pp < np && no === "%") {
-            return !util.shouldFlatten(po, no);
+            return po === "+" || po === "-";
           }
 
-          // Add parenthesis when working with binary operators
+          // Add parenthesis when working with bitwise operators
           // It's not stricly needed but helps with code understanding
           if (util.isBitwiseOperator(po)) {
             return true;

tests/binary-expressions/__snapshots__/jsfmt.spec.js.snap

@@ -101,6 +101,7 @@ a ** -b;
 -(a**b);
 (a * b) ** c;
 a ** (b * c);
+(a % b) ** c;
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 a ** (b ** c);
 (a ** b) ** c;
@@ -110,6 +111,7 @@ a ** -b;
 -(a ** b);
 (a * b) ** c;
 a ** (b * c);
+(a % b) ** c;
 
 `;
 

tests/binary-expressions/exp.js

@@ -6,3 +6,4 @@ a ** -b;
 -(a**b);
 (a * b) ** c;
 a ** (b * c);
+(a % b) ** c;