New Intention Action: Replace with operator assign

Replaces a statement of the form a = a + b with a statement of the form
a += b.

idea/testData/intentions/replaceWithOperatorAssign/invalidSubtraction.kt

@@ -0,0 +1,5 @@
+// IS_APPLICABLE: false
+fun foo() {
+    var x = 0
+    <caret>x = 1 - x
+}