cmd/compile: extract inline related fields into separate Inline type

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>

src/cmd/compile/internal/gc/main.go

@@ -544,7 +544,7 @@ func Main(archInit func(*Arch)) {
 		// Typecheck imported function bodies if debug['l'] > 1,
 		// otherwise lazily when used or re-exported.
 		for _, n := range importlist {
-			if n.Func.Inl.Len() != 0 {
+			if n.Func.Inl != nil {
 				saveerrors()
 				typecheckinl(n)
 			}