Allow setting stack values in analysis frames

Adds a method `setStack` to analysis.Frame which allows setting the
dataflow value of a stack slot.

Could be used in nullness analysis. After, for example, an instance call,
aliases of the receiver can be set to NotNull, both in locals and on the
stack.

asm-analysis/src/main/java/org/objectweb/asm/tree/analysis/Frame.java

@@ -178,6 +178,17 @@ public class Frame<V extends Value> {
     return values[nLocals + index];
   }
 
+  /**
+   * Sets the value of the given stack slot.
+   *
+   * @param index the index of an operand stack slot.
+   * @param value the new value of the stack slot.
+   * @throws IndexOutOfBoundsException if the stack slot does not exist.
+   */
+  public void setStack(final int index, final V value) throws IndexOutOfBoundsException {
+    values[index + nLocals] = value;
+  }
+
   /** Clears the operand stack of this frame. */
   public void clearStack() {
     nStack = 0;