spirv: Make VertexIndex and VertexId both non-zero-based

GLSL has gl_VertexID which is supposed to be non-zero-based.

SPIR-V has both VertexIndex and VertexId builtins whose meanings are
defined by the APIs.

Vulkan defines VertexIndex as being non-zero-based. In Vulkan VertexId
and InstanceId have no meaning and are pretty much just reserved for
OpenGL at this point.

GL_ARB_spirv removes VertexIndex and defines VertexId to be the same
as gl_VertexId (which is also non-zero-based).

Previously in Mesa it was treating VertexIndex as non-zero-based and
VertexId as zero-based, so it was breaking for GL. This behaviour was
apparently based on Khronos bug 14255. However that bug doesn’t seem
to have made a final decision for VertexId.

Assuming there really is no other definition for VertexId for Vulkan
it seems better to just make them both have the same value.

v2: update comment and commit descriptions, based on Jason Ekstrand
    explanation of the meaning/rationale behind all those builtins
    (Jason)

Reviewed-by: Jason Ekstrand <[email protected]>

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