cmd/asm: refactor some encoding functions for load/store with immediate offset on...

cmd/asm: refactor some encoding functions for load/store with immediate offset on arm64

Some of the current functions for encoding load/store with
immediate offset instructions, like opstr12(), opstr9(),
opldr12(), opldr9() and opldrpp(), etc., they have the same
code, so this patch refactors them and merges them into two
functions opstr() and opldr().

Change-Id: I60367f8b720b77c7ebe6d66905a950dcf7701836
Reviewed-on: https://go-review.googlesource.com/c/go/+/263479
Run-TryBot: fannie zhang <Fannie.Zhang@arm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: fannie zhang <Fannie.Zhang@arm.com>