cmd/compile: don't free the Prog list if we look at it after flush

Only tests do this, provide them a hook to disable freeing
after flush.

Change-Id: I810c6c51414a93f476a18ba07b807e16092bf8cf
Reviewed-on: https://go-review.googlesource.com/19907Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/asm/internal/asm/endtoend_test.go

@@ -179,7 +179,7 @@ Diff:
 		t.Errorf(format, args...)
 		ok = false
 	}
-	obj.Flushplist(ctxt)
+	obj.FlushplistNoFree(ctxt)
 
 	for p := top; p != nil; p = p.Link {
 		if p.As == obj.ATEXT {

src/cmd/internal/obj/objfile.go

@@ -116,6 +116,12 @@ func Writeobjdirect(ctxt *Link, b *Biobuf) {
 }
 
 func Flushplist(ctxt *Link) {
+	flushplist(ctxt, true)
+}
+func FlushplistNoFree(ctxt *Link) {
+	flushplist(ctxt, false)
+}
+func flushplist(ctxt *Link, freeProgs bool) {
 	var flag int
 	var s *LSym
 	var p *Prog
@@ -318,7 +324,9 @@ func Flushplist(ctxt *Link) {
 	ctxt.Plist = nil
 	ctxt.Plast = nil
 	ctxt.Curp = nil
-	ctxt.freeProgs()
+	if freeProgs {
+		ctxt.freeProgs()
+	}
 }
 
 func Writeobjfile(ctxt *Link, b *Biobuf) {