i965: Move is_math/is_tex/is_control_flow() to backend_instruction.

i965: Move is_math/is_tex/is_control_flow() to backend_instruction.

These are entirely based on the opcode, which is available in
backend_instruction.  It makes sense to only implement them in one
place.

This changes the VS implementation of is_tex() slightly, which now
accepts FS_OPCODE_TXB and SHADER_OPCODE_LOD.  However, since those
aren't generated in the VS anyway, it should be fine.

This also makes is_control_flow() available in the VS.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>