… shifts on Go < 1.13

CL 337529 introduced upfront type-checking of constant shift operands,
to avoid converting their type to uint (per the spec). However, it
had an oversight in that the checks intended for non-constant operands
still ran after the explicit checking of constant operands. As a
result, there are at least two bugs:
 - When GoVersion is < 1.13, we report spurious errors for untyped
   constant shift operands.
 - When the operand is an untyped float constant, we still convert to
   uint (this was a known bug reported in #47410).

Looking at this now, it seems clear that we can avoid both of these bugs
by simply not running the additional checks in the case of a constant
operand. However, this should be considered with some care, as shifts
are notoriously tricky.

While cherry-picking, the new test file is updated to use the go1_12
package name, following our convention for specifying language version
in the release branch.

Fixes #52032

Change-Id: Ia489cc5470b92a8187d3de0423d05b309daf47bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/396775
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit 8a816d5)
Reviewed-on: https://go-review.googlesource.com/c/go/+/397680