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.

enum
{
    LLVMnone,
    LLVMintrinsic,
    LLVMno_typeinfo,
    LLVMno_moduleinfo,
    LLVMalloca,
    LLVMva_start,
    LLVMva_copy,
    LLVMva_end,
    LLVMva_arg,
    LLVMinline_asm
};