FIR: report UNSAFE_CALL on dot when possible

compiler/testData/diagnostics/tests/smartCasts/loops/whileTrivial.fir.kt

@@ -1,11 +0,0 @@
-fun x(): Boolean { return true }
-
-public fun foo(p: String?): Int {
-    // Like whileTrue but 2 == 2 is in use
-    while(2 == 2) {
-        p!!.length
-        if (x()) break
-    }
-    // Smart cast should not work in this case, see KT-6284
-    return p.<!UNSAFE_CALL!>length<!>
-}