cmd/internal/obj/asm64: add support for moving BITCON to RSP

Constant of BITCON type can be moved into RSP by MOVD or MOVW instructions
directly, this CL enables this format of these two instructions.

For 32-bit ADDWop instructions with constant, rewrite the high 32-bit
to be a repetition of the low 32-bit, just as ANDWop instructions do,
so that we can optimize ADDW $bitcon, Rn, Rt as:
MOVW $bitcon, Rtmp
ADDW Rtmp, Rn, Rt
The original code is:
MOVZ $bitcon_low, Rtmp
MOVK $bitcon_high,Rtmp
ADDW Rtmp, Rn, Rt

Change-Id: I30e71972bcfd6470a8b6e6ffbacaee79d523805a
Reviewed-on: https://go-review.googlesource.com/c/go/+/289649


Trust: eric fang <eric.fang@arm.com>
Run-TryBot: eric fang <eric.fang@arm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: eric fang <eric.fang@arm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>