s390: Bug fix in insn selection

s390: Bug fix in insn selection

The bug was introduced in b3830b82f4 and exposed by GCC when compiling
with -g (without -O2). Here's the thing:

      if (expr->Iex.Binop.op == Iop_Add64) {
--->     ULong disp = arg2->Iex.Const.con->Ico.U64;
         if (arg2->tag == Iex_Const && ulong_fits_signed_20bit(disp)) {
            h1 = s390_isel_int_expr(env, arg1);

This possibly segfaults when arg2 is not a constant.
Optimising GCC figures it can first check arg2->tag == Iex_Const
which is cheaper than disp = arg2->Iex.Const.con->Ico.U64;
Nice one.

Regtested with both default compiler flags and -g only.