[release-branch.go1.15] cmd/compile: fix left shift constant folding rule
The 32-bit left shift constant folding rule should keep its result
properly sign extended.
Fixes #41720
Fixes #41711
Change-Id: I0fc74444d444274e911952e1725dab0b7737a846
Reviewed-on: https://go-review.googlesource.com/c/go/+/258817
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org> Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>