prefer instanceOf instead of getClass() (netty#9366)

Motivation:

`instanceOf` doesn't perform null check like `getClass()` does. So `instanceOf` may be faster. However, it not true for all cases, as C2 could eliminate these null checks for `getClass()`.

Modification:

Replaced `string.getClass() == AsciiString.class` with `string instanceof AsciiString`.

Proof:

```
@BenchmarkMode(Mode.Throughput)
@fork(value = 1)
@State(Scope.Thread)
@WarmUp(iterations = 5, time = 1, batchSize = 1000)
@measurement(iterations = 10, time = 1, batchSize = 1000)
public class GetClassInstanceOf {

    Object key;

    @setup
    public void setup() {
        key = "123";
    }

    @benchmark
    public boolean getClassEquals() {
        return key.getClass() == String.class;
    }

    @benchmark
    public boolean instanceOf() {
        return key instanceof String;
    }

}
```

```
Benchmark                           Mode  Cnt       Score      Error  Units
GetClassInstanceOf.getClassEquals  thrpt   10  401863.130 ± 3092.568  ops/s
GetClassInstanceOf.instanceOf      thrpt   10  421386.176 ± 4317.328  ops/s
```

common/src/main/java/io/netty/util/AsciiString.java

@@ -477,7 +477,7 @@ public AsciiString concat(CharSequence string) {
             return this;
         }
 
-        if (string.getClass() == AsciiString.class) {
+        if (string instanceof AsciiString) {
             AsciiString that = (AsciiString) string;
             if (isEmpty()) {
                 return that;
@@ -530,7 +530,7 @@ public boolean contentEqualsIgnoreCase(CharSequence string) {
             return false;
         }
 
-        if (string.getClass() == AsciiString.class) {
+        if (string instanceof AsciiString) {
             AsciiString rhs = (AsciiString) string;
             for (int i = arrayOffset(), j = rhs.arrayOffset(); i < length(); ++i, ++j) {
                 if (!equalsIgnoreCase(value[i], rhs.value[j])) {
@@ -989,7 +989,7 @@ public AsciiString toUpperCase() {
      * @return a new string with characters {@code <= \\u0020} removed from the beginning and the end.
      */
     public static CharSequence trim(CharSequence c) {
-        if (c.getClass() == AsciiString.class) {
+        if (c instanceof AsciiString) {
             return ((AsciiString) c).trim();
         }
         if (c instanceof String) {
@@ -1044,7 +1044,7 @@ public boolean contentEquals(CharSequence a) {
         if (a == null || a.length() != length()) {
             return false;
         }
-        if (a.getClass() == AsciiString.class) {
+        if (a instanceof AsciiString) {
             return equals(a);
         }
 
@@ -1393,7 +1393,7 @@ public boolean equals(CharSequence a, CharSequence b) {
      * {@link AsciiString}, just returns the same instance.
      */
     public static AsciiString of(CharSequence string) {
-        return string.getClass() == AsciiString.class ? (AsciiString) string : new AsciiString(string);
+        return string instanceof AsciiString ? (AsciiString) string : new AsciiString(string);
     }
 
     /**
@@ -1417,7 +1417,7 @@ public static int hashCode(CharSequence value) {
         if (value == null) {
             return 0;
         }
-        if (value.getClass() == AsciiString.class) {
+        if (value instanceof AsciiString) {
             return value.hashCode();
         }
 
@@ -1447,10 +1447,10 @@ public static boolean contentEqualsIgnoreCase(CharSequence a, CharSequence b) {
             return a == b;
         }
 
-        if (a.getClass() == AsciiString.class) {
+        if (a instanceof AsciiString) {
             return ((AsciiString) a).contentEqualsIgnoreCase(b);
         }
-        if (b.getClass() == AsciiString.class) {
+        if (b instanceof AsciiString) {
             return ((AsciiString) b).contentEqualsIgnoreCase(a);
         }
 
@@ -1509,11 +1509,11 @@ public static boolean contentEquals(CharSequence a, CharSequence b) {
             return a == b;
         }
 
-        if (a.getClass() == AsciiString.class) {
+        if (a instanceof AsciiString) {
             return ((AsciiString) a).contentEquals(b);
         }
 
-        if (b.getClass() == AsciiString.class) {
+        if (b instanceof AsciiString) {
             return ((AsciiString) b).contentEquals(a);
         }