go/types: remove superfluous code for shift checking

Negative constant shift counts are already handled
earlier in the code. No need anymore for this extra
section.

With this change, the shift code matches types2
with respect to the function logic.

Change-Id: Ic8b7f382271c79ab66021e30955cd9bac092332b
Reviewed-on: https://go-review.googlesource.com/c/go/+/303093


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>