Place OpVariable at the beginning of the block

Place OpVariable at the beginning of the block

SPIR-V requires that all OpVariable instructions in a function
must be in the first block in the function.

VK-GL-CTS issue: 1382

Components: Vulkan, Framework

Affects:
dEQP-VK.spirv_assembly.instruction.compute.loop_control.*
dEQP-VK.spirv_assembly.instruction.compute.8bit_storage.*
dEQP-VK.spirv_assembly.instruction.compute.16bit_storage.*
dEQP-VK.spirv_assembly.instruction.compute.pointer_parameter.*

Change-Id: Ib012eec83ccd130a7d6d586e3fdb529b908c21b3
(cherry picked from commit 7273686be36aeb14f7f86077782c4141a74931e3)