[FIR] Fix inconsistent RETURN_TYPE_MISMATCH and TYPE_MISMATCH reporting on functions and properties

^KT-51203 Fixed

compiler/testData/diagnostics/tests/smartCasts/varnotnull/varIntNull.fir.kt

@@ -1,5 +1,5 @@
 fun foo(): Int {
     var i: Int? = 42
     i = null
-    return <!RETURN_TYPE_MISMATCH, TYPE_MISMATCH!>i + 1<!>
+    return <!RETURN_TYPE_MISMATCH!>i + 1<!>
 }