cmd/compile: simplify the implementation of LoweredZeroLoop on loong64

Removes 6484 instructions from the go binary on loong64.

            before     after    delta
asm         561517     561225   -292
cgo         480929     480493   -436
compile    2887121    2886277   -844
cover       530429     530125   -304
fix         851649     851093   -556
link        728361     727813   -548
preprofile  240713     240545   -168
vet         824869     824297   -572
go         1638645    1636053   -2592
gofmt       320213     320041   -172

Change-Id: I15da6e94a4166aed03373657fa4785063aa32f35
Reviewed-on: https://go-review.googlesource.com/c/go/+/754000
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Meidan Li <limeidan@loongson.cn>