Fix 16bit_storage dependency in some spirv instruction fp16 tests

Fix 16bit_storage dependency in some spirv instruction fp16 tests

Some tests were ignoring the m_useStorageExt flag when creating the
shaders. Other tests were always saving the fp16 results in a fp16 SSBO
when it is not actually needed.

Finally added more conversion tests that don't need 16bit_storage.

Components: Vulkan
VK-GL-CTS issue: 1877

Added:

   dEQP-VK.spirv_assembly.instruction.*.fconvert.float64_to_float16_no_storage*

Modified:

   dEQP-VK.spirv_assembly.instruction.*float16*

Change-Id: I14897cc30a9ba2eec05ed71b25bca22ed5ef832d