cmd/compile: for location lists, handle case where prev block is not a pred

Before this change, location list construction would extend
from the previous (in linear order) block, even if was not a
flow predecessor.  This can cause a debugger to tell lies.

Fix accounts for this in block merging code by (crudely)
"changing" all variables live from a previous block if it
is not also a predecessor.

Fixes #28486.

Change-Id: I11336b0b969f0cd09f40f4e5f2bdfdeb02f377a4
Reviewed-on: https://go-review.googlesource.com/c/146718


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>