fix broken dominance caught by ssacheck

See https://go-review.googlesource.com/c/go/+/483875 for more details.

This happens when sccp generates a temporary Value v217 and it attempts
to apply rewrite rule on it to see if v217 can be fold to constant
value,

v227 = Const64 <int32>
v117 = Const32 <int32> [3]
v217 = Mul32 <int32> v117 v227

In this case, v217 can not match any rules in generic rules, so it left
in entry block while its arguments lives other blocks, which breaks the
dominance relationship. These temporary values are dead values and will
be removed in further deadcode phase but checkFunc catches them because
it runs right after every pass.

The proposed fix is to invalidate unmatched temporary value immediately.

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

@@ -358,8 +358,10 @@ func computeLattice(f *Func, val *Value, args ...*Value) lattice {
 			return lattice{constant, constValue}
 		}
 	}
-	// Either we can not match generic rules for given value or it does not satisfy
-	// additional constraints(e.g. divide by zero)
+	// Either we can not match generic rules for given value or it does not
+	// satisfy additional constraints(e.g. divide by zero), in these cases, clean
+	// up temporary value immediately in case they are not dominated by their args.
+	constValue.reset(OpInvalid)
 	return lattice{bottom, nil}
 }