Cache array length in for-in-array loop if possible

If the range expression is not a local variable (which can be updated in
the loop body affecting loop behavior, see KT-21354), we can cache the
array length, thus turning a for-in-array loop into a simple optimizable
counter loop.

 #KT-21321 In Progress

compiler/testData/codegen/box/ranges/forInArrayWithArrayVarUpdatedInLoopBody.kt

@@ -0,0 +1,23 @@
+// WITH_RUNTIME
+// IGNORE_BACKEND: JS
+
+// In Kotlin 1.0, in a for-in-array loop, range expression is cached in
+// a local variable unless it is already a local variable.
+// This caused the following quirky behavior:
+// if an array variable is updated in the loop body, it affects the loop
+// execution (see https://youtrack.jetbrains.com/issue/KT-21354).
+// Most likely it is a bug, however, fixing it right now is a breaking
+// change requiring a proper deprecation loop.
+// When the design decision is made, it might be required to update this
+// test (e.g., by adding a proper LANGUAGE_VERSION directive).
+// Note that JS back-end handles this case "correctly".
+
+fun box(): String {
+    var xs = intArrayOf(1, 2, 3)
+    var sum = 0
+    for (x in xs) {
+        sum = sum * 10 + x
+        xs = intArrayOf(4, 5)
+    }
+    return if (sum == 15) "OK" else "Fail: $sum"
+}