cmd/compile: always use StackMapDontCare as register map index when reg map is not...

cmd/compile: always use StackMapDontCare as register map index when reg map is not used

When go115ReduceLiveness is true (so we don't emit actual
register maps), use StackMapDontCare consistently for the
register map index, so RegMapValid is always false.

This fixes a compiler crash when doing -live=2 debug print.

Fixes #39251.

Change-Id: Ice087af491fa69c413f8ee59f923b72d592c0643
Reviewed-on: https://go-review.googlesource.com/c/go/+/235418
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>