stream: avoid function call where possible

Instead of assigning a boolean, move the function call that assigns a
value to the boolean to the only place that boolean is checked. This
avoids the function call in cases where it is not needed.

Refs: nodejs#41488 (review)

lib/internal/streams/utils.js

@@ -118,17 +118,19 @@ function isReadableFinished(stream, strict) {
 
 function isReadable(stream) {
   if (stream && stream[kIsReadable] != null) return stream[kIsReadable];
-  const r = isReadableNodeStream(stream);
   if (typeof stream?.readable !== 'boolean') return null;
   if (isDestroyed(stream)) return false;
-  return r && stream.readable && !isReadableFinished(stream);
+  return isReadableNodeStream(stream) &&
+    stream.readable &&
+    !isReadableFinished(stream);
 }
 
 function isWritable(stream) {
-  const r = isWritableNodeStream(stream);
   if (typeof stream?.writable !== 'boolean') return null;
   if (isDestroyed(stream)) return false;
-  return r && stream.writable && !isWritableEnded(stream);
+  return isWritableNodeStream(stream) &&
+    stream.writable &&
+    !isWritableEnded(stream);
 }
 
 function isFinished(stream, opts) {