cmd/compile: add "surprised by IData of Arg" case for register args

This fixes a compile crash for
GOEXPERIMENT=regabi,regabiargs go test -c  go/constant

Updates #40724.

Change-Id: I238cef436e045647815326fc8fdb025c30ba1f5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/307309


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>