cmd/compile: simplify the support for 32bit high multiply on loong64

cmd/compile: simplify the support for 32bit high multiply on loong64

Removes 152 instructions from the Go binary on loong64.

Change-Id: Icf8ead4f4ca965f51add85ac5e45c3cca8916401
Reviewed-on: https://go-review.googlesource.com/c/go/+/700335
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>