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/if.kt

@@ -0,0 +1,10 @@
+// !DIAGNOSTICS: -UNUSED_VALUE
+
+fun foo(f: Boolean): Int {
+    val i: Int
+    if (f) {
+        i = 1
+    }
+    <!VAL_REASSIGNMENT!>i<!> = 3
+    return i
+}