Revert "cmd/compile: de-virtualize interface calls"

This reverts commit 4e0c7c3f61.

Reason for revert: The presence-of-optimization test program is fragile, breaks under noopt, and might break if the Go libraries are tweaked.  It needs to be (re)written without reference to other packages.

Change-Id: I3aaf1ab006a1a255f961a978e9c984341740e3c7
Reviewed-on: https://go-review.googlesource.com/38097
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -1684,6 +1684,7 @@ func structargs(tl *Type, mustname bool) []*Node {
 //	rcvr - U
 //	method - M func (t T)(), a TFIELD type struct
 //	newnam - the eventual mangled name of this function
+
 func genwrapper(rcvr *Type, method *Field, newnam *Sym, iface int) {
 	if false && Debug['r'] != 0 {
 		fmt.Printf("genwrapper rcvrtype=%v method=%v newnam=%v\n", rcvr, method, newnam)
@@ -1719,7 +1720,6 @@ func genwrapper(rcvr *Type, method *Field, newnam *Sym, iface int) {
 	fn.Func.Nname = newname(newnam)
 	fn.Func.Nname.Name.Defn = fn
 	fn.Func.Nname.Name.Param.Ntype = t
-	fn.Func.Nname.Sym.SetExported(true) // prevent export; see closure.go
 	declare(fn.Func.Nname, PFUNC)
 	funchdr(fn)
 
@@ -1923,14 +1923,6 @@ func implements(t, iface *Type, m, samename **Field, ptr *int) bool {
 		}
 	}
 
-	// We're going to emit an OCONVIFACE.
-	// Call itabname so that (t, iface)
-	// gets added to itabs early, which allows
-	// us to de-virtualize calls through this
-	// type/interface pair later. See peekitabs in reflect.go
-	if isdirectiface(t0) && !iface.IsEmptyInterface() {
-		itabname(t0, iface)
-	}
 	return true
 }