Check for REGSP use before checking for invalid instructions. This simplifies
the code and avoids duplicated op generation.
Change-Id: I302e3b54a29742f2c48024b25162200f5966cfaa
Reviewed-on: https://go-review.googlesource.com/c/go/+/461138
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>