cmd/compile: fix array case in types-for-register parameter

Corrected typo/thinko.

We should keep the test for this, but it doesn't run yet because of reflection
as far as I know (but I am not testing w/ GOEXPERIMENT).

See https://github.com/golang/go/issues/44816#issuecomment-805297295

Updates #40724
Updates #44816

Change-Id: Ia12d0d4db00a8ec7174e72de460173876bd17874
Reviewed-on: https://go-review.googlesource.com/c/go/+/304233


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>