Introduce FirInitializerTypeMismatchChecker

compiler/testData/diagnostics/tests/when/kt9929.fir.kt

@@ -1,5 +1,5 @@
 // !WITH_NEW_INFERENCE
-val test: Int = if (true) {
+val test: Int = <!INITIALIZER_TYPE_MISMATCH!>if (true) {
     when (2) {
         1 -> 1
         else -> null
@@ -7,4 +7,4 @@ val test: Int = if (true) {
 }
 else {
     2
-}
+}<!>

compiler/testData/diagnostics/tests/when/kt9972.fir.kt

@@ -21,9 +21,9 @@ fun test1(): Int {
 }
 
 fun test2(): Int {
-    val x: String = when {
+    val x: String = <!INITIALIZER_TYPE_MISMATCH!>when {
                         true -> Any()
                         else -> null
-                    } ?: return 0
+                    } ?: return 0<!>
     return x.hashCode()
 }