i965/vec4: Extract depctrl hazards

i965/vec4: Extract depctrl hazards

Move this to a separate function so that we can begin to add other little
caveats without making too big a mess.

NOTE: There is some desire to improve this function eventually, but we need to
fix a bug first.

v2:
Use const for the inst for the hazard check (Matt)
Invert safe logic to get rid of the double negative (Matt)
Add PRM reference for predicates (Matt)
Add note about empirical evidence for math (Matt)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>