nir: unroll some loops with a variable limit

nir: unroll some loops with a variable limit

For some loops can have a single terminator but the exact trip
count is still unknown. For example:

   for (int i = 0; i < imin(x, 4); i++)
      ...

Shader-db results radeonsi (all affected are from Tropico 5):

Totals from affected shaders:
SGPRS: 144 -> 152 (5.56 %)
VGPRS: 124 -> 108 (-12.90 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 5180 -> 6640 (28.19 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 17 -> 21 (23.53 %)
Wait states: 0 -> 0 (0.00 %)

Shader-db results i965 (SKL):

total loops in shared programs: 3808 -> 3802 (-0.16%)
loops in affected programs: 6 -> 0
helped: 6
HURT: 0

vkpipeline-db results RADV (Unrolls some Skyrim VR shaders):

Totals from affected shaders:
SGPRS: 304 -> 304 (0.00 %)
VGPRS: 296 -> 292 (-1.35 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 15756 -> 25884 (64.28 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 29 -> 29 (0.00 %)
Wait states: 0 -> 0 (0.00 %)

v2: fix bug where last iteration would get optimised away by
    mistake.

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