cmd/compile: fix possible nil deref added in CL 270943

In the event allocSpan returned a nil, this would crash.
Cleaned up the code and comments slightly, too.

Change-Id: I6231d4b4c14218e6956b4a97a205adc3206f59ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/316429


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>