cmd/compile: use constant zero register instead of specialized zero instructions on mips64x
Refer to CL 633075, mips64x has a constant zero register that can be used to do this.
Change-Id: I7b60f9a9fe0015299f48b9219ba0eddd3c02e07a
Reviewed-on: https://go-review.googlesource.com/c/go/+/700935
Auto-Submit: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Mark Freeman <markfreeman@google.com>