cmd/internal/obj/arm64: factor out code generation for addition of 12 bit immediates
Factor out and simplify code that generates the addition of a 12 bit immediate
(the addition of a negative value is still handled via subtraction). This also
fixes the mishandling of the case where v is 0.
Change-Id: I6040f33d2fec87b772272531b3bf02390ae7f200
Reviewed-on: https://go-review.googlesource.com/c/go/+/461141
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Eric Fang <eric.fang@arm.com> Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au> Reviewed-by: Cherry Mui <cherryyz@google.com>