cmd/compile: add Type.Elem

This removes almost all direct access to
Type’s heavily overloaded Type field.

Mostly generated by eg, manually checked.

Significant manual changes:

* reflect.go's typPkg used Type indiscriminately.
  Use it only for specific etypes.
* gen.go's visitComponents contained a usage of Type
  with structs. Using Type for structs no longer
  occurs, and the Fatal contained therein has not triggered,
  so it has been axed.
* Scary code in cgen.go's cgen_slice is now explicitly scary.

Passes toolstash -cmp.

Change-Id: I2dbfb3c959da7ae239f964d83898c204affcabc6
Reviewed-on: https://go-review.googlesource.com/21331
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -39,7 +39,7 @@ func fnpkg(fn *Node) *Pkg {
 		rcvr := fn.Type.Recv().Type
 
 		if Isptr[rcvr.Etype] {
-			rcvr = rcvr.Type
+			rcvr = rcvr.Elem()
 		}
 		if rcvr.Sym == nil {
 			Fatalf("receiver with no sym: [%v] %v  (%v)", fn.Sym, Nconv(fn, FmtLong), rcvr)
@@ -747,7 +747,7 @@ func mkinlcall1(n *Node, fn *Node, isddd bool) *Node {
 			as.Right = nodnil()
 			as.Right.Type = varargtype
 		} else {
-			vararrtype := typArray(varargtype.Type, int64(varargcount))
+			vararrtype := typArray(varargtype.Elem(), int64(varargcount))
 			as.Right = Nod(OCOMPLIT, nil, typenod(vararrtype))
 			as.Right.List.Set(varargs)
 			as.Right = Nod(OSLICE, as.Right, Nod(OKEY, nil, nil))
@@ -866,7 +866,7 @@ func retvar(t *Field, i int) *Node {
 // when they come from a multiple return call.
 func argvar(t *Type, i int) *Node {
 	n := newname(LookupN("~arg", i))
-	n.Type = t.Type
+	n.Type = t.Elem()
 	n.Class = PAUTO
 	n.Used = true
 	n.Name.Curfn = Curfn // the calling function, not the called one