cmd/compile: remove superfluous SetBounded call

cmd/compile: remove superfluous SetBounded call

The call does nothing when applied to an OLSH node.
It would be unnecessary anyway, since we're shifting by a small constant.

Passes toolstash-check.

Change-Id: If858711f1704f44637fa0f6a4c66cbaad6db24b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/228699
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>