Do not cache "last" value (i.e., size/length) in lowered for-loop

iteration over CharSequences.

CharSequences may be mutable (e.g., StringBuilder) and therefore its
contents and length can change within the loop.

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

@@ -0,0 +1,16 @@
+// KJS_WITH_FULL_RUNTIME
+// WITH_RUNTIME
+import kotlin.test.*
+
+fun box(): String {
+    val sb = StringBuilder("1234")
+    val result = StringBuilder()
+    for (c in sb) {
+        sb.clear()
+        result.append(c)
+    }
+    assertEquals("", sb.toString())
+    assertEquals("1", result.toString())
+
+    return "OK"
+}

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

@@ -0,0 +1,19 @@
+// KJS_WITH_FULL_RUNTIME
+// WITH_RUNTIME
+import kotlin.test.*
+
+fun box(): String {
+    val sb = StringBuilder("1234")
+    val result = StringBuilder()
+    var ctr = 0
+    for (c in sb) {
+        if (ctr % 2 == 0)
+            sb.append('x')
+        ctr++
+        result.append(c)
+    }
+    assertEquals("1234xxxx", sb.toString())
+    assertEquals("1234xxxx", result.toString())
+
+    return "OK"
+}