go.tools/ssa: use correct names for Captures.

Until now, the name of the captured ssa.Value, not
types.Var was used, leading to confusing disassembly
when it was a numbered register.  See:
https://code.google.com/p/go/issues/detail?id=6337

Now the output is:

# Free variables:
#   0:	a *int
#   1:	b *int
func func@6.9() int:
.0.entry:
        t0 = *b
        t1 = *a
        t2 = *b
        etc...

BUG=6337

R=crawshaw
CC=golang-dev
https://golang.org/cl/13249049

ssa/func.go

@@ -430,7 +430,7 @@ func (f *Function) lookup(obj types.Object, escaping bool) Value {
 	}
 	outer := f.Enclosing.lookup(obj, true) // escaping
 	v := &Capture{
-		name:   outer.Name(),
+		name:   obj.Name(),
 		typ:    outer.Type(),
 		pos:    outer.Pos(),
 		outer:  outer,