[FIR] Add DFA implications when one side of an Elvis operator is null

When one side of an Elvis operator can only be `null`, and the entire
Elvis operator expression cannot be `null`, this implies that the
opposite side of the Elvis operator cannot be `null`. Add such
implications to the Elvis exit node of the DFA. This helps smart-casting
of variables used within long Elvis operator chains.

#KT-49249 Fixed

compiler/fir/analysis-tests/testData/resolve/smartcasts/longElvisChain.kt

@@ -6,12 +6,27 @@ fun test_1() {
     val b: Unit? = null
     val c = a ?: b?.let { return it } ?: return
     c<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
-    throw <!TYPE_MISMATCH!>a<!>
+    throw a
 }
 
 fun test_2() {
     val a: Throwable? = null;
     val b: Unit? = null
     val c = a ?: b?.let { return it } ?: return
-    throw <!TYPE_MISMATCH!>a<!>
+    throw a
+}
+
+fun test_3() {
+    val a: Throwable? = null;
+    val b: Unit? = null
+    val c = b?.let { return it } ?: a ?: return
+    c<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
+    throw a
+}
+
+fun test_4() {
+    val a: Throwable? = null;
+    val b: Unit? = null
+    val c = b?.let { return it } ?: a ?: return
+    throw a
 }