i965: expose BRW_OPCODE_[F32TO16/F16TO32] name on gen8+

i965: expose BRW_OPCODE_[F32TO16/F16TO32] name on gen8+

Technically those hw operations are only available on gen7, as gen8+
support the conversion on the MOV. But, when using the builder to
implement nir operations (example: nir_op_fquantize2f16), it is not
needed to do the gen check. This check is done later, on the final
emission at brw_F32TO16 (brw_eu_emit), choosing between the MOV or the
specific operation accordingly.

So in the middle, during optimization phases those hw operations can
be around for gen8+ too.

Without this patch, several (at least 95) vulkan-cts quantize tests
crashes when using INTEL_DEBUG=optimizer. For example:
dEQP-VK.spirv_assembly.instruction.graphics.opquantize.too_small_vert

v2: simplify the code using GEN_GE (Ilia Mirkin)
v3: tweak brw_instruction_name instead of changing opcode_descs
    table, that is used for validation (Matt Turner)

Reviewed-by: Matt Turner <mattst88@gmail.com>