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

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
-}
+}