[FIR] Elvis operator: flexible rhs type implies flexible type of whole elvis

It's required for further removing of enhancement of final static fields

Also, it's k2-potential feature

^KT-62467 Fixed

compiler/fir/analysis-tests/testData/resolve/inference/flexibleTypeAtRightSideOfElvis.fir.txt

@@ -0,0 +1,28 @@
+FILE: main.kt
+    public final fun testWithStatic1(x1: R|kotlin/String?|): R|kotlin/String| {
+        lval x: R|kotlin/String?|
+        R|<local>/x| = R|<local>/x1| ?: Q|JavaClass|.R|/JavaClass.X*s|
+        ^testWithStatic1 R|<local>/x|
+    }
+    public final fun testWithStatic2(x1: R|kotlin/String?|): R|kotlin/String| {
+        lvar x: R|kotlin/String?| = Null(null)
+        R|<local>/x| = R|<local>/x1| ?: Q|JavaClass|.R|/JavaClass.f*s|()
+        ^testWithStatic2 R|<local>/x|
+    }
+    public final fun testWithStatic3(x1: R|kotlin/String?|): R|kotlin/String| {
+        lval x: R|kotlin/String?| = R|<local>/x1| ?: Q|JavaClass|.R|/JavaClass.X*s|
+        ^testWithStatic3 R|<local>/x|
+    }
+    public final fun testWithStatic4(x1: R|kotlin/String?|, w: R|kotlin/Int|): R|kotlin/String| {
+        lvar x: R|kotlin/String?|
+        R|<local>/x| = R|<local>/x1| ?: when (R|<local>/w|) {
+            ==($subj$, Int(42)) ->  {
+                Q|JavaClass|.R|/JavaClass.X*s|
+            }
+            else ->  {
+                Q|JavaClass|.R|/JavaClass.f*s|()
+            }
+        }
+
+        ^testWithStatic4 R|<local>/x|
+    }

compiler/fir/analysis-tests/testData/resolve/inference/flexibleTypeAtRightSideOfElvis.kt

@@ -0,0 +1,36 @@
+// ISSUE: KT-62467
+
+// FILE: JavaClass.java
+
+public class JavaClass {
+    public static String X = null;
+    public static String f() { return null; }
+}
+
+// FILE: main.kt
+
+fun testWithStatic1(x1: String?): String {
+    val x: String?
+    x = x1 ?: JavaClass.X
+    return x
+}
+
+fun testWithStatic2(x1: String?): String {
+    var x: String? = null
+    x = x1 ?: JavaClass.f()
+    return x
+}
+
+fun testWithStatic3(x1: String?): String {
+    val x: String? = x1 ?: JavaClass.X
+    return <!RETURN_TYPE_MISMATCH!>x<!>
+}
+
+fun testWithStatic4(x1: String?, w: Int): String {
+    var x: String?
+    x = x1 ?: when (w) {
+        42 -> JavaClass.X
+        else -> JavaClass.f()
+    }
+    return x
+}