cmd/compile: not use REGTMP in ZeroRange on ARM64

cmd/compile: not use REGTMP in ZeroRange on ARM64

For async preemption, we will be using REGTMP as a temporary
register in injected call on ARM64, which will clobber it. So any
code that uses REGTMP is not safe for async preemption.

For ZeroRange, which is inserted at the function entry where
there is no register live, we could just use a different register
and avoid REGTMP.

Change-Id: I3db763828df6846908c9843a9912597efb9efcdf
Reviewed-on: https://go-review.googlesource.com/c/go/+/203458
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>