nir: add opt_if_loop_terminator()

nir: add opt_if_loop_terminator()

This pass detects potential loop terminators and moves intructions
from the non breaking branch after the if-statement.

This enables both the new opt_if_simplification() pass and loop
unrolling to potentially progress further.

Unexpectedly this change speed up shader-db run times by ~3%

Ivy Bridge shader-db results (all changes in dolphin/ubershaders):

total instructions in shared programs: 9995662 -> 9995338 (-0.00%)
instructions in affected programs: 87845 -> 87521 (-0.37%)
helped: 27
HURT: 0

total cycles in shared programs: 230931495 -> 230925015 (-0.00%)
cycles in affected programs: 56391385 -> 56384905 (-0.01%)
helped: 27
HURT: 0

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>