cmd/compile: coalesce some 386 shift lowering rules
cmd/compile: coalesce some 386 shift lowering rules

No changes to generated rule matching code.

Change-Id: I0009146a6be30e151a403c4801c03abf0646703d
Reviewed-on: https://go-review.googlesource.com/c/go/+/216999
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>

cloudflare/go,akutz/go,CAFxX/go,nawawi/go,akutz/go,cloudflare/go,golang/go,golang/go,golang/go,nawawi/go,akutz/go,nawawi/go,akutz/go,nawawi/go,golang/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,akutz/go,akutz/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,akutz/go,akutz/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go