Subgroup and workgroup reconvergence tests.

Subgroup and workgroup reconvergence tests.

These tests run shaders with randomly-generated control
flow and use subgroup operations to check whether subgroups
reconverge when control flow merges. The non-experimental
tests exercise the
VK_KHR_shader_subgroup_uniform_control_flow extension,
and the experimental tests go further and exercise maximal
reconvergence (and also increase the number of tests).

New Tests: dEQP-VK.reconvergence.*
New Tests: dEQP-VK-experimental.reconvergence.*
Component: Vulkan

Change-Id: I83136ad1aca6daece4a612a58d93aa175a2c5184
(cherry picked from commit 09f8fc2e7ecbbf97a2a30eeb4c6ce2c4e0ecb160)