[FE 1.0] Correctly set USED_AS_EXPRESSION for unreachable when and if blocks

^KT-50028 Fixed

compiler/testData/ir/irText/expressions/kt50028.kt

@@ -0,0 +1,17 @@
+// ISSUE: KT-50028
+
+fun test_1(): String {
+    return when {
+        else -> {
+            return ""
+        }
+    }
+}
+
+fun test_2(b: Boolean): Boolean {
+    return if (b) {
+        true
+    } else {
+        throw NotImplementedError()
+    }
+}