freedreno/ir3: encode instruction category in opc_t

freedreno/ir3: encode instruction category in opc_t

Been on my TODO list for a while.  If nothing else this will make gdb
properly grok the opc_t enum.

This first step preserves ir3_instruction::category (with an added
assert that category matches what is encoded in opc_t).  Next step is
to drop the category field (and arg to ir3_instr_create()), but that
is split into next commit for bisectability and so that we can run
piglit in the intermediate state to flush out any problems.

Signed-off-by: Rob Clark <robclark@freedesktop.org>