[dev.ssa] cmd/compile: addressed vars and closures

Cleaned up first-block-in-function code.
Added cases for |PHEAP for PPARAM and PAUTO.
Made PPARAMOUT act more like PAUTO for purposes
of address generation and vardef placement.
Added cases for OCLOSUREVAR and Ops for getting closure
pointer.  Closure ops are scheduled at top of entry block
to capture DX.

Wrote test that seems to show proper behavior for addressed
parameters, locals, and returns.

Change-Id: Iee93ebf9e3d9f74cfb4d1c1da8038eb278d8a857
Reviewed-on: https://go-review.googlesource.com/14650
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: David Chase <drchase@google.com>

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

@@ -148,7 +148,7 @@ type Param struct {
 // Func holds Node fields used only with function-like nodes.
 type Func struct {
 	Shortname  *Node
-	Enter      *NodeList
+	Enter      *NodeList // for example, allocate and initialize memory for escaping parameters
 	Exit       *NodeList
 	Cvars      *NodeList // closure params
 	Dcl        *NodeList // autodcl for this func/closure