new variant for EmitAggregateCopy


when reading the comments in CodeGenFunction::EmitAggregateCopy
there seem to be two problems:

1. "Aggregate assignment turns into llvm.memcpy. This is almost valid per
For me, "almost valid" means that it is valid for 99% of the cases but there should
be the possibility to switch it off

2. "FIXME: If we have a volatile struct, the optimizer can remove what might
appear to be `extra' memory ops"
This sounds like the current implementation can produce incorrect code for
volatile struct variables.

This is my alternative implementation based on element wise copy of all fields.
I would suggest that it is used if isVolatile is true or if a flag in TargetInfo
indicates that it always should be used. This way the 99% of the cases
continue using memcpy.

     const clang::RecordType* recordType = Ty->getAs<clang::RecordType>();

     // get record decl
     clang::RecordDecl* recordDecl = recordType->getDecl();

     // iterate over fields
     clang::CodeGen::CodeGenTypes& codeGenTypes = CGM.getTypes();
     const clang::CodeGen::CGRecordLayout& layout = codeGenTypes.getCGRecordLayout(recordDecl);
     clang::RecordDecl::field_iterator it = recordDecl->field_begin();
     clang::RecordDecl::field_iterator end = recordDecl->field_end();
     for (; it != end; ++it)
         clang::FieldDecl* fieldDecl = *it;
         int fieldIndex = layout.getLLVMFieldNo(fieldDecl);

         llvm::Value *SrcFieldPtr = Builder.CreateConstInBoundsGEP2_32(SrcPtr, 0, fieldIndex);
         llvm::Value *DestFieldPtr = Builder.CreateConstInBoundsGEP2_32(DestPtr, 0, fieldIndex);

         // check if field is an aggregate
         QualType fieldType = fieldDecl->getType();
         if (fieldType->getAs<clang::RecordType>() != NULL)
             // copy nested aggregate
             EmitAggregateCopy(DestFieldPtr, SrcFieldPtr, fieldType, isVolatile);
             // copy value
             llvm::Value *Element = Builder.CreateLoad(SrcFieldPtr, isVolatile, "tmp");
             Builder.CreateStore(Element, DestFieldPtr, isVolatile);