intel/fs: Take into account region strides during SIMD lowering decision of SHUFFLE.

intel/fs: Take into account region strides during SIMD lowering decision of SHUFFLE.

This fixes a bug in the handcrafted SIMD lowering done by the SHUFFLE
code generation, which wasn't taking into account the source and
destination region strides while deciding whether it needs to split an
instruction.

v2: Use new element_sz() helper instead of left shift. (Lionel)

Fixes: 90c9f29518d ("i965/fs: Add support for nir_intrinsic_shuffle")
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14273>