cmd/compile: absorb NEGV into branch on loong64

cmd/compile: absorb NEGV into branch on loong64

Removes 132 instructions from the go binary on loong64.

Change-Id: Ia02dc305b12f63a64f3f48d120ef852d45cc2a7b
Reviewed-on: https://go-review.googlesource.com/c/go/+/695115
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>