cmd/compile: fix an issue in MNEG of ARM64

There are two less optimized SSA rules in my previous CL
https://go-review.googlesource.com/c/go/+/95075 .

This CL fixes that issue and a test case gets about 10%
performance improvement.
name    old time/op  new time/op  delta
MNEG-4   263µs ± 3%   235µs ± 3%  -10.53%  (p=0.000 n=20+20)
(https://github.com/benshi001/ugo1/blob/master/mneg_7_test.go)

Change-Id: I30087097e281dd9d9d1c870d32e13b4ef4a96ad3
Reviewed-on: https://go-review.googlesource.com/99495


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>