cmd/compile: break apart dumptypestructs

dumptypestructs did several different jobs.
Split them into separate functions
and call them in turn.

Hand dumptypestructs a list of dcls,
rather than reading the global.

Rename dumpptabs for (marginal) clarity.

This is groundwork for compiling autogenerated
functions concurrently.

Passes toolstash-check.

Change-Id: I627a1dffc70a7e4b7b4436ab19af1406267f01dc
Reviewed-on: https://go-review.googlesource.com/41501
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -136,8 +136,11 @@ func dumpobj1(outfile string, mode int) {
 	externs := len(externdcl)
 
 	dumpglobls()
-	dumpptabs()
-	dumptypestructs()
+	addptabs()
+	dumptypestructs(externdcl)
+	dumptabs()
+	dumpimportstrings()
+	dumpbasictypes()
 
 	// Dump extra globals.
 	tmp := externdcl
@@ -171,7 +174,7 @@ func dumpobj1(outfile string, mode int) {
 	bout.Close()
 }
 
-func dumpptabs() {
+func addptabs() {
 	if !Ctxt.Flag_dynlink || localpkg.Name != "main" {
 		return
 	}

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

@@ -1394,9 +1394,9 @@ func addsignat(t *types.Type) {
 	signatlist[formalType(t)] = true
 }
 
-func dumptypestructs() {
-	// copy types from externdcl list to signatlist
-	for _, n := range externdcl {
+func dumptypestructs(dcls []*Node) {
+	// copy types from dcl list to signatlist
+	for _, n := range dcls {
 		if n.Op == OTYPE {
 			addsignat(n.Type)
 		}
@@ -1421,7 +1421,9 @@ func dumptypestructs() {
 			}
 		}
 	}
+}
 
+func dumptabs() {
 	// process itabs
 	for _, i := range itabs {
 		// dump empty itab symbol into i.sym
@@ -1474,18 +1476,22 @@ func dumptypestructs() {
 		}
 		ggloblsym(s, int32(ot), int16(obj.RODATA))
 	}
+}
 
+func dumpimportstrings() {
 	// generate import strings for imported packages
 	for _, p := range types.ImportedPkgList() {
 		dimportpath(p)
 	}
+}
 
+func dumpbasictypes() {
 	// do basic types if compiling package runtime.
 	// they have to be in at least one package,
 	// and runtime is always loaded implicitly,
 	// so this is as good as any.
 	// another possible choice would be package main,
-	// but using runtime means fewer copies in .6 files.
+	// but using runtime means fewer copies in object files.
 	if myimportpath == "runtime" {
 		for i := types.EType(1); i <= TBOOL; i++ {
 			dtypesym(types.NewPtr(types.Types[i]))