Inl, Inldcl, and InlCost are only applicable to functions with bodies
that can be inlined, so pull them out into a separate Inline type to
make understanding them easier.

A side benefit is that we can check if a function can be inlined by
just checking if n.Func.Inl is non-nil, which simplifies handling of
empty function bodies.

While here, remove some unnecessary Curfn twiddling, and make imported
functions use Inl.Dcl instead of Func.Dcl for consistency for local
functions.

Passes toolstash-check.

Change-Id: Ifd4a80349d85d9e8e4484952b38ec4a63182e81f
Reviewed-on: https://go-review.googlesource.com/104756
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>