This extends fetch_payload_reg() to support fetching vector registers
like barycentrics stored on the payload as a contiguous sequence of
SIMD-wide vectors. In the SIMD32 case, both halves of the SIMD16
vector registers specified as regs[0] and regs[1] are zipped to
construct a single SIMD32-wide vector.
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26606>
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