cmd/compile: correct maintain use count when phi args merge

The critical phase did not correctly maintain the use count
when two predecessors of a new critical block transmit the
same value.

Change-Id: Iba802c98ebb84e36a410721ec32c867140efb6d4
Reviewed-on: https://go-review.googlesource.com/20822
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Todd Neal <todd@tneal.org>

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

@@ -79,6 +79,7 @@ func critical(f *Func) {
 			if reusedBlock {
 				d.Preds = append(d.Preds, c)
 				b.Preds[i] = nil
+				phi.Args[i].Uses--
 				phi.Args[i] = nil
 			} else {
 				// splice it in