VM incorrectly infers that value is non-nullable after 'is' check

[alexmarkov]

Similar to #40792, but now in VM's optimizer.

import "package:expect/expect.dart";

dynamic result;

bar(x) {
  result = (x == null) ? 'null' : 'not null';
}

class A<S, T extends S> {
  @pragma('vm:never-inline')
  foo(S x) {
    if (x is T) {
      bar(x);
    }
  }
}

main(List<String> args) {
  for (int i = 0; i < 30001; ++i) {
    result = 'unexpected';
    new A<Null, Null>().foo(null);
    Expect.equals('null', result);
  }
}

out/ReleaseX64/dart foo.dart
Unhandled exception:
Expect.equals(expected: <null>, actual: <not null>) fails.
#0      Expect._fail (package:expect/expect.dart:682:5)
#1      Expect.equals (package:expect/expect.dart:123:7)
#2      main (file:///.../foo.dart:23:12)
#3      _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:299:32)
#4      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)