cmd/compile: fix buglet in inlined info abstract function dwarf-gen

When generating DWARF inlined info records, it's possible to have a
local function whose only callsites are inlined away, meaning that we
emit an abstract function DIE but no regular subprogram DIE. When
emitting DWARF scope info we need to handle this case (specifically
when scoping PCs, check for the case that the func in question has
been entirely deleted).

Fixes #44344.

Change-Id: I9f5bc692f225aa4c5c23f7bd2e50bcf7fe4fc5f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/293309


TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>