KT-6916: do not create Progression instances in for-in-downTo loops

compiler/testData/codegen/bytecodeText/forLoop/forIntInDownTo.kt

@@ -0,0 +1,15 @@
+// WITH_RUNTIME
+
+fun test(): Int {
+    var sum = 0
+    for (i in 4 downTo 1) {
+        sum = sum * 10 + i
+    }
+    return sum
+}
+
+// 0 iterator
+// 0 getStart
+// 0 getEnd
+// 0 getFirst
+// 0 getLast

compiler/testData/codegen/bytecodeText/forLoop/primitiveProgression.kt

@@ -1,7 +1,8 @@
 fun f() {
     for (i in 0..5 step 2) {
     }
-    for (i in 5 downTo 1) {
+
+    for (i in 5 downTo 1 step 1) { // suppress optimized code generation for 'for-in-downTo'
     }
 }