cmd/compile: combine multiple rotate instructions

cmd/compile: combine multiple rotate instructions

Rotating by c, then by d, is the same as rotating by c+d.

Change-Id: I36df82261460ff80f7c6d39bcdf0e840cef1c91a
Reviewed-on: https://go-review.googlesource.com/c/go/+/424894
Reviewed-by: Wayne Zuo <wdvxdr@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ruinan Sun <Ruinan.Sun@arm.com>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>