cmd/compile: on ARM64 merge SRA into TBZ & TBNZ

Change-Id: I9596dbca8991c93c7543d10dc1b155056dfa7db3
Reviewed-on: https://go-review.googlesource.com/c/go/+/759500
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@google.com>