vulkan: Use larger workgroups for mul_mat_vec when M is small (llama/15355)

vulkan: Use larger workgroups for mul_mat_vec when M is small (llama/15355)

* vulkan: Use larger workgroups for mul_mat_vec when M is small

Also use subgroup instructions for (part of) the reduction when supported.
Without this, the more expensive reductions would eat into the benefits of
the larger workgroups.

* update heuristic for amd/intel

Co-authored-by: 0cc4m <redacted>
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Co-authored-by: 0cc4m <redacted>