Retain data flow info after array accesses

compiler/testData/diagnostics/tests/dataFlowInfoTraversal/ArrayIndices.kt

@@ -0,0 +1,9 @@
+fun foo1(a: Int?, b: Array<Array<Int>>) {
+    b[a!!][a<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>] = a
+    a : Int
+}
+
+fun foo2(a: Int?, b: Array<Array<Int>>) {
+    b[0][a!!] = a
+    a : Int
+}