emitRetUT: cope with arguments overlapping with results (#3546)

emitRetUT: cope with arguments overlapping with results (#3546)

In decodeFloat_Int# we have the C-- code:

    mp_tmp1  = Sp - WDS(1);
    mp_tmp_w = Sp - WDS(2);

    /* arguments: F1 = Float# */
    arg = F1;

    /* Perform the operation */
    foreign "C" __decodeFloat_Int(mp_tmp1 "ptr", mp_tmp_w "ptr", arg) [];

    /* returns: (Int# (mantissa), Int# (exponent)) */
    RET_NN(W_[mp_tmp1], W_[mp_tmp_w]);

Which all looks quite reasonable.  The problem is that RET_NN() might
assign the results to the stack (with an unregisterised back end), and
in this case the arguments to RET_NN() refer to the same stack slots
that will be assigned to.

The code generator should do the right thing here, but it wasn't - it
was assuming that it could assign the results sequentially.  A 1-line
fix to use emitSimultaneously rather than emitStmts (plus comments).