go/types, types2: add an additional shift test case

The extra test just confirms that the type-checker internally
agrees with the spec with the (otherwise invisible) type given
to an untyped constant in a specific shift expression.

For #48422.

Change-Id: I6d98045f90bd20b0cc0a96a147bec9701039cb07
Reviewed-on: https://go-review.googlesource.com/c/go/+/350410


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>