[asm-cherry-pick] Allow setting stack values in analysis frames

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

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.

Author: lrytz

Diff for: src/main/java/scala/tools/asm/tree/analysis/Frame.java

@@ -179,7 +179,21 @@ public V getStack(final int index) {
     return values[nLocals + index];
   }
 
-  /** Clears the operand stack of this frame. */
+  /**
+   * Sets the value of the given stack slot.
+   *
+   * @param i
+   *   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 i, final V value) throws IndexOutOfBoundsException {
+      values[i + nLocals] = value;
+  }
+
+    /** Clears the operand stack of this frame. */
   public void clearStack() {
     nStack = 0;
   }