cmd/compile: fix mips zerorange implementation

cmd/compile: fix mips zerorange implementation

A +8 was missed when simplifying this code in CL 700936.

Fixes #75477

Change-Id: Ic7b83322dc1373432b44f0b63607141195220380
Reviewed-on: https://go-review.googlesource.com/c/go/+/703937
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Julian Zhu <jz531210@gmail.com>
Auto-Submit: Keith Randall <khr@google.com>
Reviewed-by: Tianon Gravi <admwiggin@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>