Also, fix condition for TypeTestAlwaysDiverges warning message.

This used to also include nulls but that makes no sense: null.isInstanceOf
does not diverge, it just returns false. The ok-tostring test shows
that always false should not yield a warning, since this might
come from inlined code.

Author: odersky

compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala

@@ -257,7 +257,7 @@ object TypeTestsCasts {
             if expr.tpe.isNotNull then constant(expr, Literal(Constant(true)))
             else expr.testNotNull
           else {
-            if expr.tpe.isBottomType then
+            if expr.tpe.hasClassSymbol(defn.NothingClass) then
               report.warning(TypeTestAlwaysDiverges(expr.tpe, testType), tree.srcPos)
             val nestedCtx = ctx.fresh.setNewTyperState()
             val foundClsSyms = effectiveClassSymbols(expr.tpe.widen)