cmd/compile: make amd64 version of zerorange regabi-friendly

Change the amd64 version of 'zerorange' to avoid using RAX/RDI, since
it can be called in a context when one of these registers is live
(contains an incoming parameter).

Updates #40724.

Change-Id: Ibfa2b4e156b876354d4f8bd04eb8773c7056d948
Reviewed-on: https://go-review.googlesource.com/c/go/+/305829


Trust: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>