Optimize for-in-string loops

For-in-string loop can be generated using specialized 'length' and
'charAt' method calls, and with cached string length.
Note that update of the string variable in loop body doesn't affect
loop execution semantics.

 #KT-21322 Fixed Target versions 1.2.20

compiler/tests/org/jetbrains/kotlin/codegen/BytecodeTextTestGenerated.java

@@ -1301,6 +1301,12 @@ public class BytecodeTextTestGenerated extends AbstractBytecodeTextTest {
             doTest(fileName);
         }
 
+        @TestMetadata("forInStringSpecialized.kt")
+        public void testForInStringSpecialized() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/forLoop/forInStringSpecialized.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("forInUntil.kt")
         public void testForInUntil() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/forLoop/forInUntil.kt");