Don't force line break in empty loop bodies (#1815)

This makes them consistent with empty function bodies.

Fixes https://github.com/prettier/prettier/issues/1356

src/printer.js

@@ -672,6 +672,9 @@ function genericPrintNoParens(path, options, print, args) {
           parent.type === "FunctionDeclaration" ||
           parent.type === "ObjectMethod" ||
           parent.type === "ClassMethod" ||
+          parent.type === "ForStatement" ||
+          parent.type === "WhileStatement" ||
+          parent.type === "DoWhileStatement" ||
           (parent.type === "CatchClause" && !parentParent.finalizer))
       ) {
         return "{}";

tests/flow/binding/__snapshots__/jsfmt.spec.js.snap

@@ -629,14 +629,12 @@ function try_scope() {
 
 function for_scope_let() {
   let a: number = 0;
-  for (let a = "" /* ok: local to init */; ; ) {
-  }
+  for (let a = "" /* ok: local to init */; ; ) {}
 }
 
 function for_scope_var() {
   var a: number = 0;
-  for (var a = "" /* error: string ~> number */; ; ) {
-  }
+  for (var a = "" /* error: string ~> number */; ; ) {}
 }
 
 function for_in_scope_let(o: Object) {

tests/flow/refi/__snapshots__/jsfmt.spec.js.snap

@@ -591,8 +591,7 @@ var tests = [
   function() {
     let x: { foo: ?string } = { foo: null };
     if (!x.foo) {
-      while (false) {
-      }
+      while (false) {}
       x.foo = "foo";
     }
     (x.foo: string);
@@ -601,8 +600,7 @@ var tests = [
   function() {
     let x: { foo: ?string } = { foo: null };
     if (!x.foo) {
-      for (var i = 0; i < 0; i++) {
-      }
+      for (var i = 0; i < 0; i++) {}
       x.foo = "foo";
     }
     (x.foo: string);

tests/for/__snapshots__/jsfmt.spec.js.snap

@@ -74,10 +74,8 @@ for (var i = 0; i < 10; ++i) {}
 for (;;) 0;
 for (var i = 0; i < 10; ++i) 0;
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-for (;;) {
-}
-for (var i = 0; i < 10; ++i) {
-}
+for (;;) {}
+for (var i = 0; i < 10; ++i) {}
 
 for (;;) 0;
 for (var i = 0; i < 10; ++i) 0;
@@ -98,10 +96,8 @@ async function f() {
 }
 for (a in b) 0;
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-for ((x in a); ; ) {
-}
-for (a = (a in b); ; ) {
-}
+for ((x in a); ; ) {}
+for (a = (a in b); ; ) {}
 for (let a = (b in c); ; );
 for (a && (b in c); ; );
 for (a => (b in c); ; );

tests/label/__snapshots__/jsfmt.spec.js.snap

@@ -8,8 +8,7 @@ exports[`comment.js 1`] = `
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 {
   // goto emulation
-  inf_leave: for (;;) {
-  }
+  inf_leave: for (;;) {}
 }
 
 `;

tests/while/__snapshots__/jsfmt.spec.js.snap

@@ -23,10 +23,8 @@ while (
   someVeryLongStringB &&
   someVeryLongStringC &&
   someVeryLongStringD
-) {
-}
-do {
-} while (
+) {}
+do {} while (
   someVeryLongStringA &&
   someVeryLongStringB &&
   someVeryLongStringC &&