[dev.ssa] cmd/compile: remember names of values

For debugging, spill values to named variables instead of autotmp_
variables if possible.  We do this by keeping a name -> value map
for each function, keep it up-to-date during deadcode elim, and use
it to override spill decisions in stackalloc.

It might even make stack frames a bit smaller, as it makes it easy
to identify a set of spills which are likely not to interfere.

This just works for one-word variables for now.  Strings/slices
will be a separate CL.

Change-Id: Ie89eba8cab16bcd41b311c479ec46dd7e64cdb67
Reviewed-on: https://go-review.googlesource.com/16336
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/export_test.go

@@ -6,7 +6,6 @@ package ssa
 
 import (
 	"cmd/internal/obj"
-	"fmt"
 	"testing"
 )
 
@@ -29,7 +28,7 @@ type DummyFrontend struct {
 func (DummyFrontend) StringData(s string) interface{} {
 	return nil
 }
-func (DummyFrontend) Auto(t Type) fmt.Stringer {
+func (DummyFrontend) Auto(t Type) GCNode {
 	return nil
 }