cmd/compile/internal/ssa: skip TestNexting/gdb-dbg-i22558

This test fails frequently in the longtest builder, and the failures
on the build dashboard have masked two other regressions so far.
Let's skip it until it can be fixed.

Updates #31263

Change-Id: I82bae216ebc3c5fd395c27c72c196334a130af7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/172423


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>