Normal shift rules plus constant folding are enough to generate
efficient shift-by-constant instructions.

Add test to make sure we don't generate comparisons for constant
shifts.

TODO: there are still constant shift rules on PPC64. If they
are removed, the constant folding rules are not enough to remove
all the test and mask stuff for constant shifts. Leave them in
for now.

Fixes #20663.

Change-Id: I724cc324aa8607762d0c8aacf9bfa641bda5c2a1
Reviewed-on: https://go-review.googlesource.com/60330
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>