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/nonAssignmentExpression.kt

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