st/mesa,tgsi: use enum tgsi_opcode

st/mesa,tgsi: use enum tgsi_opcode

Need to update the tgsi code and st_glsl_to_tgsi code at the same time
to prevent compile break since C++ is much pickier about implicit
enum/unsigned casting.

Bump size of glsl_to_tgsi_instruction::op to 10 bits to be sure to
avoid MSVC signed enum overflow issue.  No change in class size.

Reviewed-by: Eric Anholt <eric@anholt.net>