Update AccessInstance::verifyResult() to support 64-bit types

Update AccessInstance::verifyResult() to support 64-bit types

When 64-bit support was added to the
dEQP-VK.robustness.buffer_access.through_pointers.* tests in
https://gerrit.khronos.org/#/c/5835/ the AccessInstance::verifyResult()
function was not updated to take the new 8 byte element sizes.
It assumed all accesses were in units of 4 bytes, which doesn't
work for 64-bit types which have accesses in units of 8 bytes.

Affects:

dEQP-VK.robustness.buffer_access.through_pointers.*

Components: Vulkan

VK-GL-CTS issue: 2670

Change-Id: I4656376f127bc4cf6eeaa80ed71aaa3f265e0aac