i965: Add a pass to remove dead control flow.

i965: Add a pass to remove dead control flow.

Removes IF/ENDIF and IF/ELSE/ENDIF with no intervening instructions.

total instructions in shared programs: 1360393 -> 1360387 (-0.00%)
instructions in affected programs:     157 -> 151 (-3.82%)

(no change in vertex shaders)

Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>