VK_KHR_variable_pointers: Don't use OpPtrAccessChain on a bare SSBO

VK_KHR_variable_pointers: Don't use OpPtrAccessChain on a bare SSBO

You can't use OpPtrAccessChain on the direct result of an OpVariable
because it's not the element of any array.  Instead, we need to do our
OpPtrAccessChain on a pointer to an actual array element and that
pointer needs to be properly decorated with a stride.

Component: Vulkan

VK-GL-CTS issue: https://gitlab.khronos.org/Tracker/vk-gl-cts/issues/686

Affects:
dEQP-VK.spirv_assembly.instruction.graphics.variable_pointers.multi_buffer_*

Change-Id: I9c45fb1320099e404672ee169a6a8e5c41eb13f7