Fix for 16 to 8-bit storage spirv_assembly tests

Fix for 16 to 8-bit storage spirv_assembly tests

The output buffer was being made smaller than it was required to
hold all the results. Robust buffer accesss prevented the writes
from trampling memory after the buffer, but could mean that the
data in the valid part of the buffer was incorrectly written instead.

This change makes the output buffer large enough to prevent the
out-of-bounds writes.

Components: Vulkan

VK-GL-CTS issue: 4126

Affected tests:
dEQP-VK.spirv_assembly.instruction.graphics.8bit_storage.storagebuffer_int_16_to_8.*

Change-Id: I59145be8012f7a31037ee3fee4d7ac96a198bc00