Control-Flow Analysis: Reuse pseudo-value information for unused expression analysis

compiler/testData/diagnostics/tests/regressions/Jet69.kt

@@ -6,5 +6,5 @@ fun Any.equals(other : Any?) : Boolean = this === other
 
 fun main(args: Array<String>) {
     val command = parse("")
-    if (command == null) 1 // error on this line, but must be OK
-}
+    if (command == null) <!UNUSED_EXPRESSION!>1<!> // error on this line, but must be OK
+}

compiler/testData/diagnostics/tests/regressions/kt847.kt

@@ -1,3 +1,4 @@
+// !DIAGNOSTICS: -UNUSED_EXPRESSION
 fun <T> T.mustBe(t : T) {
-    <!UNUSED_EXPRESSION!>"$this must be$<!SYNTAX!>as<!>$t"<!>
-}
+    "$this must be$<!SYNTAX!>as<!>$t"
+}