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/whiletrue.kt

@@ -0,0 +1,11 @@
+// !DIAGNOSTICS: -UNUSED_VALUE
+
+fun foo(): Int {
+    val i: Int
+    var j = 0
+    while (true) {
+        <!VAL_REASSIGNMENT!>i<!> = ++j
+        if (j > 5) break
+    } 
+    return i
+}