[dev.ssa] cmd/compile/internal/ssa: don't treat vardef/varkill as reads

This makes deadstore elimination work reasonably again.

Change-Id: I3a8caced71f12dfb6c1d0c68b7a7d8d7a736ea23
Reviewed-on: https://go-review.googlesource.com/14536Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -30,7 +30,7 @@ func dse(f *Func) {
 				for _, a := range v.Args {
 					if a.Block == b && a.Type.IsMemory() {
 						storeUse.add(a.ID)
-						if v.Op != OpStore && v.Op != OpZero {
+						if v.Op != OpStore && v.Op != OpZero && v.Op != OpVarDef && v.Op != OpVarKill {
 							// CALL, DUFFCOPY, etc. are both
 							// reads and writes.
 							loadUse.add(a.ID)