cmd/compile: allow earlier GC of freed constant value

Minor fix, because it's the right thing to do.
No significant impact.

Change-Id: I2138285d397494daa9a88c414149c2a7860edd7e
Reviewed-on: https://go-review.googlesource.com/38001
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/ssa/func.go

@@ -140,6 +140,7 @@ func (f *Func) freeValue(v *Value) {
 		for i, cv := range vv {
 			if v == cv {
 				vv[i] = vv[len(vv)-1]
+				vv[len(vv)-1] = nil
 				f.constants[v.AuxInt] = vv[0 : len(vv)-1]
 				break
 			}