Optimize 64-bit compares with zero

src/IceInstX86BaseImpl.h

 //===- subzero/src/IceInstX86BaseImpl.h - Generic X86 instructions -*- C++ -*=//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 2074 matching lines @@
   static const typename InstX86Base<
       Machine>::Traits::Assembler::GPREmitterAddrOp AddrEmitter = {
       &InstX86Base<Machine>::Traits::Assembler::test,
       &InstX86Base<Machine>::Traits::Assembler::test};
   if (const auto SrcVar0 = llvm::dyn_cast<Variable>(Src0)) {
     if (SrcVar0->hasReg()) {
       emitIASRegOpTyGPR<Machine>(Func, Ty, SrcVar0, Src1, RegEmitter);
       return;
     }
   }
-  llvm_unreachable("Nothing actually generates this so it's untested");
   emitIASAsAddrOpTyGPR<Machine>(Func, Ty, Src0, Src1, AddrEmitter);
 }
 
 template <class Machine>
 void InstX86Test<Machine>::dump(const Cfg *Func) const {
   if (!BuildDefs::dump())
     return;
   Ostream &Str = Func->getContext()->getStrDump();
   Str << "test." << this->getSrc(0)->getType() << " ";
   this->dumpSources(Func);
@@ skipping 1204 matching lines @@
     return;
   Ostream &Str = Func->getContext()->getStrDump();
   Str << "IACA_END";
 }
 
 } // end of namespace X86Internal
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICEINSTX86BASEIMPL_H