cmd/compile/internal/gc: cleanup esc.go

* convert important functions to methods
* rename EscXXX to XXX in NodeEscState
* rename local variables more friendly
* simplify redundant code
* update comments

Change-Id: I8442bf4f8dde84523d9a2ad3d04b1cd326bd4719
Reviewed-on: https://go-review.googlesource.com/30893
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -330,8 +330,8 @@ func (n *Node) jconv(s fmt.State, flag FmtFlag) {
 		fmt.Fprintf(s, " esc(%d)", n.Esc)
 	}
 
-	if e, ok := n.Opt().(*NodeEscState); ok && e.Escloopdepth != 0 {
-		fmt.Fprintf(s, " ld(%d)", e.Escloopdepth)
+	if e, ok := n.Opt().(*NodeEscState); ok && e.Loopdepth != 0 {
+		fmt.Fprintf(s, " ld(%d)", e.Loopdepth)
 	}
 
 	if c == 0 && n.Typecheck != 0 {