[SYCL] Fix accessor to subset of buffer indexing (compiler part)
To get correct offset in subscript operator we should use range of
original buffer. To keep accessor functionality like "get_range" and "get_count"
spec conformant we should use accessor's own range.
Introduced AccessRange and MemRange to accessor implemetation. This
fields can be initialized through __init method.
Signed-off-by: Vladimir Lazarev <email@example.com>
Signed-off-by: Mariya Podchishchaeva <firstname.lastname@example.org>