cmd/compile: change Func.{Dcl,Inldcl} from NodeList to slice

A slice uses less memory than a NodeList, and has better memory locality
when walking the list.

This uncovered a tricky case involving closures: the escape analysis
pass when run on a closure was appending to the Dcl list of the OCLOSURE
rather than the ODCLFUNC.  This happened to work because they shared the
same NodeList.  Fixed with a change to addrescapes, and a check to
Tempname to catch any recurrences.

This removes the last use of the listsort function outside of tests.
I'll send a separate CL to remove it.

Unfortunately, while this passes all tests, it does not pass toolstash
-cmp.  The problem is that cmpstackvarlt does not fully determine the
sort order, and the change from listsort to sort.Sort, while generally
desirable, produces a different ordering.  I could stage this by first
making cmpstackvarlt fully determined, but no matter what toolstash -cmp
is going to break at some point.

In my casual testing the compiler is 2.2% faster.

Update #14473.

Change-Id: I367d66daa4ec73ed95c14c66ccda3a2133ad95d5
Reviewed-on: https://go-review.googlesource.com/19919
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -2564,15 +2564,15 @@ func (p *parser) stmt() *Node {
 		stmt := Nod(ORETURN, nil, nil)
 		stmt.List = results
 		if stmt.List == nil && Curfn != nil {
-			for l := Curfn.Func.Dcl; l != nil; l = l.Next {
-				if l.N.Class == PPARAM {
+			for _, ln := range Curfn.Func.Dcl {
+				if ln.Class == PPARAM {
 					continue
 				}
-				if l.N.Class != PPARAMOUT {
+				if ln.Class != PPARAMOUT {
 					break
 				}
-				if l.N.Sym.Def != l.N {
-					Yyerror("%s is shadowed during return", l.N.Sym.Name)
+				if ln.Sym.Def != ln {
+					Yyerror("%s is shadowed during return", ln.Sym.Name)
 				}
 			}
 		}