Make host code not executing "loop idiom"

"loop idiom" pass will be executed after "SYCL arguments flattening"
pass for host.

Author: yu810226

lib/CodeGen/BackendUtil.cpp

@@ -646,7 +646,12 @@ void EmitAssemblyHelper::CreatePasses(legacy::PassManager &MPM,
   // choke current Xilinx xocc compiler, the EnableLoopIdiom flag here is set
   // to false for not executing "loop idiom" pass when compiling souce code
   // of SYCL kernel.
-  if (LangOpts.SYCLIsDevice)
+  // And Since argument promotion can only be done with GEP instructions that
+  // only used by loads. If llvm.memcpy intrinsics, there will always be a
+  // cast instruction before to cast the pointer, so for the host code, the
+  // EnableLoopIdiom flag here force skipping "loop idiom" pass. The pass will
+  // be executed after "SYCL arguments flattening" pass.
+  if (LangOpts.SYCL)
     PMBuilder.EnableLoopIdiom = false;
   PMBuilder.populateModulePassManager(MPM);
 }