[FIR] Implement checker for exhaustive when's in expression position

compiler/testData/diagnostics/tests/smartCasts/kt27221_irrelevantClasses.fir.kt

@@ -10,7 +10,7 @@ object CC : C()
 fun foo(a: A) {
     if (a is B) {
         if (a is C) {
-            val t = when (a) {
+            val t = <!NO_ELSE_IN_WHEN!>when<!> (a) {
                 is CC -> "CC"
             }
         }
@@ -20,9 +20,9 @@ fun foo(a: A) {
 fun foo2(a: A) {
     if (a is C) {
         if (a is B) {
-            val t = when (a) {
+            val t = <!NO_ELSE_IN_WHEN!>when<!> (a) {
                     is CC -> "CC"
             }
         }
     }
-}
+}

compiler/testData/diagnostics/tests/smartCasts/loops/elvisIfBreakInsideWhileTrue.fir.kt

@@ -0,0 +1,9 @@
+public fun foo(x: String?, y: String?): Int {
+    while (true) {
+        x ?: <!INVALID_IF_AS_EXPRESSION!>if<!> (y == null) break
+        // y is nullable if x != null
+        y<!UNSAFE_CALL!>.<!>length
+    }
+    // y is null because of the break
+    return y<!UNSAFE_CALL!>.<!>length
+}

compiler/testData/diagnostics/tests/smartCasts/loops/elvisIfBreakInsideWhileTrue.kt

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 public fun foo(x: String?, y: String?): Int {
     while (true) {
         x ?: if (y == null) break
@@ -7,4 +6,4 @@ public fun foo(x: String?, y: String?): Int {
     }
     // y is null because of the break
     return y<!UNSAFE_CALL!>.<!>length
-}
+}