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/syntax.go

@@ -461,7 +461,6 @@ type Func struct {
 	Exit      Nodes
 	Cvars     Nodes   // closure params
 	Dcl       []*Node // autodcl for this func/closure
-	Inldcl    Nodes   // copy of dcl for use in inlining
 
 	// Parents records the parent scope of each scope within a
 	// function. The root scope (0) has no parent, so the i'th
@@ -484,8 +483,7 @@ type Func struct {
 	Nname      *Node
 	lsym       *obj.LSym
 
-	Inl     Nodes // copy of the body for use in inlining
-	InlCost int32
+	Inl *Inline
 
 	Label int32 // largest auto-generated label in this function
 
@@ -502,6 +500,15 @@ type Func struct {
 	nwbrCalls *[]nowritebarrierrecCallSym
 }
 
+// An Inline holds fields used for function bodies that can be inlined.
+type Inline struct {
+	Cost int32 // heuristic cost of inlining this function
+
+	// Copies of Func.Dcl and Nbody for use during inlining.
+	Dcl  []*Node
+	Body []*Node
+}
+
 // A Mark represents a scope boundary.
 type Mark struct {
 	// Pos is the position of the token that marks the scope
@@ -737,6 +744,11 @@ const (
 // a slice to save space.
 type Nodes struct{ slice *[]*Node }
 
+// asNodes returns a slice of *Node as a Nodes value.
+func asNodes(s []*Node) Nodes {
+	return Nodes{&s}
+}
+
 // Slice returns the entries in Nodes as a slice.
 // Changes to the slice entries (as in s[i] = n) will be reflected in
 // the Nodes.