New Intention Action: Replace with traditional assign.

This takes an expression of the form a += b and converts it to an
expression form a = a + b.

idea/testData/intentions/replaceWithTraditionalAssignment/complexRightExpression.kt

@@ -0,0 +1,6 @@
+fun foo() {
+    var x = 0
+    val a = 1
+    val b = 1
+    <caret>x += a / b
+}