Add test with unreachable switch merge inside loop

Add test with unreachable switch merge inside loop

According to VK-GL-CTS issue #952, when a switch statement is inside an
endless loop and the switch merge block is unreachable because every
case jumps to the end of the loop, at least one driver goes into an
infinite loop when the switch merge block is labeled with OpUnreachable.

New test:
dEQP-VK.spirv_assembly.instruction.compute.opunreachable.unreachable-switch-merge-in-loop

Components: Vulkan
VK-GL-CTS issue: 952

Change-Id: I8afb50ab43f29f9bdd543a5c4a45f5fe3941cd04