cmd/compile: constant fold SHLxconst of a constant on amd64
These rules fire in particular when comparing to a constant
string of length two. They should trigger even more after CL 220499.
file before after Δ %
compile
20639976 20635880 -4096 -0.020%
total
116003456 115999360 -4096 -0.004%
Change-Id: I21c1c02cf32d710d7a4eb12efab00f02796ccb84
Reviewed-on: https://go-review.googlesource.com/c/go/+/220694
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>