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/syntax.go

@@ -152,8 +152,8 @@ type Func struct {
 	Enter      *NodeList
 	Exit       *NodeList
 	Cvars      *NodeList // closure params
-	Dcl        *NodeList // autodcl for this func/closure
-	Inldcl     *NodeList // copy of dcl for use in inlining
+	Dcl        []*Node   // autodcl for this func/closure
+	Inldcl     []*Node   // copy of dcl for use in inlining
 	Closgen    int
 	Outerfunc  *Node
 	Fieldtrack []*Type