i965/cfg: Eliminate an empty then-branch of an if/else/endif

i965/cfg: Eliminate an empty then-branch of an if/else/endif

On BDW,

total instructions in shared programs: 8448571 -> 8448367 (-0.00%)
instructions in affected programs: 21000 -> 20796 (-0.97%)
helped: 116
HURT: 0

v2: Remove spurious attempt to combine the if_block with the (removed!)
else_block.  Suggested by Matt and Curro.  Correct the comment
describing what the new pass does.  Suggested by Matt.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>