Completely refactor accessor::iterator

Now iterators for non-ranged accessors should be almost equal to plain
pointers and only iterator dereference for a ranged accessor is a costly
operation which requires various calculations including division.

Author: AlexeySachkov

sycl/include/sycl/accessor.hpp

@@ -2030,12 +2030,11 @@ class __SYCL_SPECIAL_CLASS __SYCL_TYPE(accessor) accessor :
   bool operator!=(const accessor &Rhs) const { return !(*this == Rhs); }
 
   iterator begin() const noexcept {
-    return iterator::__get_begin(this, get_offset(),
-                                 get_offset() + get_range());
+    return iterator::__get_begin(this, getMemoryRange());
   }
 
   iterator end() const noexcept {
-    return iterator::__get_end(this, get_offset(), get_offset() + get_range());
+    return iterator::__get_end(this, getMemoryRange());
   }
 
 private: