intel/brw: Xe2+ can do SIMD16 for extended math on HF types

BSpec 56797:
Math operation rules when half-floats are used on both source and
destination operands and both source and destinations are packed.

The execution size must be 16.

Signed-off-by: Rohan Garg <rohan.garg@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27235>

src/intel/compiler/brw_fs_lower_simd_width.cpp

@@ -287,8 +287,8 @@ brw_fs_get_lowered_simd_width(const fs_visitor *shader, const fs_inst *inst)
        * The execution size must be 16.
        */
       if (is_half_float_src_dst(inst))
-         return MIN2(8,  inst->exec_size);
-
+         return devinfo->ver < 20 ? MIN2(8,  inst->exec_size) :
+                                    MIN2(16, inst->exec_size);
       return MIN2(16, inst->exec_size);
    }