cmd/compile: remove the NORconst op on mips{,64}

cmd/compile: remove the NORconst op on mips{,64}

In the mips{,64} instruction sets and their extensions, there is no
NORI instruction.

Change-Id: If008442c792297d011b3d0c1e8501e62e32ab175
Reviewed-on: https://go-review.googlesource.com/c/go/+/735900
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Cherry Mui <cherryyz@google.com>