[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

src/main/java/scala/tools/asm/tree/analysis/Frame.java

@@ -202,6 +202,20 @@ public V getStack(final int i) throws IndexOutOfBoundsException {
         return values[i + locals];
     }
 
+    /**
+     * 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 + locals] = value;
+    }
+
     /**
      * Clears the operand stack of this frame.
      */