Fix indexing a push constant array member

Fix indexing a push constant array member

gl_VertexIndex is used to index a push constant array member.
Vulkan spec says: Any member of a push constant block that is
declared as an array must only be accessed with dynamically
uniform indices. But gl_VertexIndex is not dynamically uniform.

Changed the test to use dynamically uniform indexing.

Affects:
dEQP-VK.spirv_assembly.instruction.spirv1p4.entrypoint.vert_pc_entry_point

Components: Vulkan

VK-GL-CTS issue: 2000

Change-Id: I767aacfe0fde27e083f7b4a82f808cb99fb31d16
(cherry picked from commit 30b013b2ba0dfd5eabd3f7a0412135a1c8be8038)