cmd/compile: fix for zerorange on plan9-amd64

In CL 305829 a problematic change was made to the compiler's
amd64-specific "zerorange" function. In zerorange the compiler uses
different sets of strategies depending on the size of the stack frame
it needs to zero; turns out that only on plan9-amd64 was it hitting
the final fallback strategy, which is a REPSTOSQ instruction. REPSTOSQ
takes RAX as an input, hence the changes made in CL 305829 (switching
to R13) were incorrect.

This patch restores the zerorange REPSTOSQ sequence (back to use RAX).
This is going to be an interim solution, since long term we need to
avoid touching RAX in the function prolog (since if the new register
ABI is in effect, it will hold a live value).

Fixes #45372.

Change-Id: Ic89a6a2a76d6e03b9fbda99275101e96b70fdf5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/307469


Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>