Fixed dmdfe generated class invariant calls. Also insert proper "this !is null" check before it. Fixed invariant* dstress regressions and generally seems like a good idea. Hope I didn't break anything.. minitests run fine!

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