8359454: Enhance String handling

Reviewed-by: abakhtin, fferrari
Backport-of: 2f2665738a67aeed224b54870608a346eb627d2a

Author: Aleksei Voitylov

src/java.base/share/classes/java/lang/AbstractStringBuilder.java

@@ -1313,8 +1313,8 @@ public AbstractStringBuilder insert(int dstOffset, CharSequence s,
         ensureCapacityInternal(count + len);
         shift(dstOffset, len);
         count += len;
-        if (s instanceof String) {
-            putStringAt(dstOffset, (String) s, start, end);
+        if (s instanceof String str && str.length() == len) {
+            putStringAt(dstOffset, str);
         } else {
             putCharsAt(dstOffset, s, start, end);
         }
@@ -1741,11 +1741,6 @@ private void inflateIfNeededFor(AbstractStringBuilder input) {
         }
     }
 
-    private void putStringAt(int index, String str, int off, int end) {
-        inflateIfNeededFor(str);
-        str.getBytes(value, off, index, coder, end - off);
-    }
-
     private void putStringAt(int index, String str) {
         inflateIfNeededFor(str);
         str.getBytes(value, index, coder);