Return `void*` from _d_allocclass so LLVM doesn't do weird things with it... This allows `-instcombine` followed by `-gvn` to do devirtualization, so add `-gvn` in strategic places in the default pass order.

module tangotests.volatile1;

import tango.stdc.stdlib;

void main()
{
    int var = rand();
    {
        int i = var;
        volatile;
        int j = i;
    }
}