Relax the 'phi after non-phi' SSA sanity check to allow
RegKill ops interspersed with phi ops in a block. This fixes
a sanity check failure when -dwarflocationlists is enabled.

Updates #22694.

Change-Id: Iaae604ab6f1a8b150664dd120003727a6fb2f698
Reviewed-on: https://go-review.googlesource.com/77610
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>