Fix SPIR-V assembly tests with bad cfgs

Fix SPIR-V assembly tests with bad cfgs

* Update SPIR-V assembly tests to not declare a continue target as a
merge block

Component: Vulkan

VK-GL-CTS Issue: 2122

Affects:
dEQP-VK.spirv_assembly.instruction.graphics.switch_block_order.out_of_order*
dEQP-VK.spirv_assembly.instruction.graphics.opphi.out_of_order*
dEQP-VK.spirv_assembly.instruction.graphics.loop.multi_block_loop_construct*
dEQP-VK.spirv_assembly.instruction.graphics.loop.multi_block_continue_construct*
dEQP-VK.spirv_assembly.instruction.graphics.loop.continue*
dEQP-VK.spirv_assembly.instruction.graphics.loop.break*
dEQP-VK.spirv_assembly.instruction.graphics.loop.return*

Change-Id: I370331414aae27b53ed79b448b72581e2352c3d7