cmd/compile: fold constant shifts into (SHL|SHR|SAR)Xload ops

cmd/compile: fold constant shifts into (SHL|SHR|SAR)Xload ops

We should prefer a constant shift op to a X shift op.
That way we don't have to materialize the constant to shift by.

Should fix GOAMD64=v3 builder

Change-Id: I56b45d2940c959382b970e3f962ed4a09cc2a239
Reviewed-on: https://go-review.googlesource.com/c/go/+/400254
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Wayne Zuo <wdvxdr@golangcn.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>