[FE 1.0] Fix reporting of non exhaustive when statement for whens with subject

^KT-48653 Fixed

compiler/testData/diagnostics/tests/when/kt48653_before.kt

@@ -0,0 +1,22 @@
+// FIR_IDENTICAL
+// LANGUAGE: -ProhibitNonExhaustiveWhenOnAlgebraicTypes
+// ISSUE: KT-48653
+
+sealed class Sealed {
+    object A : Sealed()
+    object B : Sealed()
+}
+fun functionReturningSealed(): Sealed = null!!
+
+fun test_1() {
+    <!NON_EXHAUSTIVE_WHEN_STATEMENT!>when<!> (val result = functionReturningSealed()) {
+        is Sealed.A -> {}
+    }
+}
+
+fun test_2() {
+    val result2 = functionReturningSealed()
+    <!NON_EXHAUSTIVE_WHEN_STATEMENT!>when<!> (result2) {
+        is Sealed.A -> {}
+    }
+}