More precise value reassignment analysis. #KT-7418 Fixed. #KT-6902 Fixed.

Variable can now be "exactly initialized", "may be initialized" and "not initialized".
A set of relevant tests. Some fixed tests.

compiler/testData/diagnostics/tests/reassignment/foronly.kt

@@ -0,0 +1,9 @@
+// !DIAGNOSTICS: -UNUSED_VALUE
+
+fun foo(k: Int): Int {
+    val i: Int
+    for (j in 1..k) {
+        <!VAL_REASSIGNMENT!>i<!> = j
+    }
+    return <!UNINITIALIZED_VARIABLE!>i<!>
+}