cmd/internal/obj/loong64: using {xv,v}slli.d to perform copying between vector registers

cmd/internal/obj/loong64: using {xv,v}slli.d to perform copying between vector registers

Go asm syntax:
	VMOVQ     Vj, Vd
	XVMOVQ    Xj, Xd

Equivalent platform assembler syntax:
	vslli.d   vd, vj, 0x0
	xvslli.d  xd, xj, 0x0

Change-Id: Ifddc3d4d3fbaa6fee2e079bf2ebfe96a2febaa1c
Reviewed-on: https://go-review.googlesource.com/c/go/+/716801
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>