cmd/internal/obj/arm64: refactor the handling of shifted RSP

Some arithmetic operation instructions such as ADD and SUB support two
formats of left shift (<<) operation, namely shifted register format and
extended register format. And the encoding, supported registers and shifted
amount are both different.

The assembly parser doesn't distinguish them and parses them into TYPE_SHIFT
type, because the parser can't tell them apart and in most cases extended
left-shift can be replaced by shifted left-shift. The only exception is
when the second source register or the destination register is RSP.

This CL converts this case into the extended format in the preprocess stage,
which helps to simplify some of the logic of the new assembler implementation
and also makes this situation look more reasonable.

Change-Id: I2cd7d2d663b38a7ba77a9fef1092708b8cb9bc3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/311709


Trust: Eric Fang <eric.fang@arm.com>
Run-TryBot: Eric Fang <eric.fang@arm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>