cmd/compile: make generated function code more consistent

There are a bunch of places where we generate functions: equality and
hash functions; method expression and promoted method wrappers; and
print/delete wrappers for defer/go statements.

This CL brings them in sync by:

1) Always using dclfunc and funcbody. Most were already using this,
but makepartialcall needed some changes.

2) Removing duplicate types.Markdcl/types.Popdcl calls. These are
already handled by dclfunc and funcbody.

3) Using structargs (already used by genwrapper) to construct new
param/result lists from existing types.

4) Always accessing the parameter ONAME nodes through Field.Nname
instead of poking into the ODCLFIELD. Also, since creating a slice of
the entire parameter list is common, extract this out into a
paramNnames helper function.

5) Add a Type.IsVariadic method to simplify identifying variadic
function types.

Passes toolstash-check -gcflags=-dwarf=false. DWARF output changes
because using structargs in makepartialcall changes the generated
parameter names.

Change-Id: I6661d3699afdbe7852ad60db5a4ec6eeb2b696e4
Reviewed-on: https://go-review.googlesource.com/108216
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/walk.go

@@ -3861,12 +3861,9 @@ func wrapCall(n *Node, init *Nodes) *Node {
 	}
 
 	t := nod(OTFUNC, nil, nil)
-	var args []*Node
 	for i, arg := range n.List.Slice() {
-		buf := fmt.Sprintf("a%d", i)
-		a := namedfield(buf, arg.Type)
-		t.List.Append(a)
-		args = append(args, a.Left)
+		s := lookupN("a", i)
+		t.List.Append(symfield(s, arg.Type))
 	}
 
 	wrapCall_prgen++
@@ -3874,7 +3871,7 @@ func wrapCall(n *Node, init *Nodes) *Node {
 	fn := dclfunc(sym, t)
 
 	a := nod(n.Op, nil, nil)
-	a.List.Set(args)
+	a.List.Set(paramNnames(t.Type))
 	a = typecheck(a, Etop)
 	fn.Nbody.Set1(a)