intel/compiler: properly size attribute wa_flags array for Vulkan

intel/compiler: properly size attribute wa_flags array for Vulkan

Mesa will map user defined vertex input attributes to slots
starting at VERT_ATTRIB_GENERIC0 which gives us room for only 16
slots (up to GL_VERT_ATTRIB_MAX). This sufficient for GL, where
we expose exactly 16 vertex attributes for user defined inputs, but
in Vulkan we can expose up to 28 (which are also mapped from
VERT_ATTRIB_GENERIC0 onwards) so we need to account for this when
we scope the size of the array of attribute workaround flags
that is used during the brw_vertex_workarounds NIR pass. This
prevents out-of-bounds accesses in that array for NIR shaders
that use more than 16 vertex input attributes.

Fixes:
dEQP-VK.pipeline.vertex_input.max_attributes.*

Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>