nir: add loop unroll support for complex wrapper loops

nir: add loop unroll support for complex wrapper loops

In GLSL IR we cheat with switch statements and simply convert them
into loops with a single iteration. This allowed us to make use of
the existing jump instruction handling provided by the loop handing
code, it also allows dead code to be cleaned up once we have
wrapped the code in a loop.

However using loops in this way created previously unrollable loops
which limits further optimisations. Here we provide a way to unroll
loops that end in a break and have multiple other exits.

All shader-db changes are from the dolphin uber shaders. There is a
small amount of HURT shaders but in general the improvements far
exceed the HURT.

shader-db results IVB:

total instructions in shared programs: 10018187 -> 10016468 (-0.02%)
instructions in affected programs: 104080 -> 102361 (-1.65%)
helped: 36
HURT: 15

total cycles in shared programs: 220065064 -> 154529655 (-29.78%)
cycles in affected programs: 126063017 -> 60527608 (-51.99%)
helped: 51
HURT: 0

total loops in shared programs: 2515 -> 2308 (-8.23%)
loops in affected programs: 903 -> 696 (-22.92%)
helped: 51
HURT: 0

total spills in shared programs: 4370 -> 4124 (-5.63%)
spills in affected programs: 1397 -> 1151 (-17.61%)
helped: 9
HURT: 12

total fills in shared programs: 4581 -> 4419 (-3.54%)
fills in affected programs: 2201 -> 2039 (-7.36%)
helped: 9
HURT: 15

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>