spirv: handle gl_SampleMask

spirv: handle gl_SampleMask

SPIR-V maps both gl_SampleMask and gl_SampleMaskIn to the same
builtin (SampleMask). The only way to tell which one we are dealing with
is to check if it is an input or an output.

Fixes:
dEQP-VK.pipeline.multisample_shader_builtin.sample_mask.write.*

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>