go/types: preserve untyped constants on the RHS of a shift expression

CL 291316 fixed go/types to verify that untyped shift counts are
representable by uint, but as a side effect also converted their types
to uint.

Rearrange the logic to keep the check for representability, but not
actually convert untyped integer constants. Untyped non-integer
constants are still converted, to preserve the behavior of 1.16. This
behavior for non-integer types is a bug, filed as #47410.

Updates #47410
Fixes #47243

Change-Id: I5eab4aab35b97f932fccdee2d4a18623ee2ccad5
Reviewed-on: https://go-review.googlesource.com/c/go/+/337529


Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>