cmd/compile: extend loong MOV*idx rules to match ADDshiftLLV

cmd/compile: extend loong MOV*idx rules to match ADDshiftLLV

Fixes #76085

I was focused on restoring the old behavior and fixing the failing
test/codegen/floats.go:index* tests.

It is probable this same bug hides elsewhere in this file.

Change-Id: Ibb2cb2be5c7bbeb5eafa9705d998a67380f2b04c
Reviewed-on: https://go-review.googlesource.com/c/go/+/715580
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>