Fix loop continue construct for SPIR-V assembly

Fix loop continue construct for SPIR-V assembly

The multi-block continue construct tests were setting the continue
target to the loop header itself, which is not valid according to the
new structural dominance rules. Set it to the first block inside the
loop instead, which still gives a multi-block continue construct but
also follows the validation rules.

Components: Vulkan
VK-GL-CTS issue: 3896
Affects:
dEQP-VK.spirv_assembly.instruction.graphics.loop.multi_block_continue_construct_*

Change-Id: I7480f6107696ec8587625bfa110aa7478308512f