interp: fix phi instruction

I've discovered a bug in the implementation of the PHI instruction in
the interp package. This commit fixes the bug.

I've found this issue while investigating an issue with maps after
running interp per package.

interp/interp_test.go

@@ -13,6 +13,7 @@ import (
 func TestInterp(t *testing.T) {
 	for _, name := range []string{
 		"basic",
+		"phi",
 		"slice-copy",
 		"consteval",
 		"map",