cmd/compile/internal/ssa/gen: fix error message for wrong arg length

When arg length is wrong, op is not set, so it always prints
"should have 0 args".

Change-Id: If7bcb41d993919d0038d2a09e16188c79dfbd858
Reviewed-on: https://go-review.googlesource.com/28831Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/gen/rulegen.go

@@ -616,7 +616,7 @@ func parseValue(val string, arch arch, loc string) (op opData, oparch string, ty
 			if strict {
 				return false
 			} else {
-				log.Printf("%s: op %s (%s) should have %d args, has %d", loc, s[0], archname, op.argLength, len(args))
+				log.Printf("%s: op %s (%s) should have %d args, has %d", loc, s[0], archname, x.argLength, len(args))
 			}
 		}
 		return true