radeonsi: eliminate trivial constant VS outputs

radeonsi: eliminate trivial constant VS outputs

These constant value VS PARAM exports:
- 0,0,0,0
- 0,0,0,1
- 1,1,1,0
- 1,1,1,1
can be loaded into PS inputs using the DEFAULT_VAL field, and the VS exports
can be removed from the IR to save export & parameter memory.

After LLVM optimizations, analyze the IR to see which exports are equal to
the ones listed above (or undef) and remove them if they are.

Targeted use cases:
- All DX9 eON ports always clear 10 VS outputs to 0.0 even if most of them
  are unused by PS (such as Witcher 2 below).
- VS output arrays with unused elements that the GLSL compiler can't
  eliminate (such as Batman below).

The shader-db deltas are quite interesting:
(not from upstream si-report.py, it won't be upstreamed)

PERCENTAGE DELTAS    Shaders PARAM exports (affected only)
batman_arkham_origins    589  -67.17 %
bioshock-infinite       1769   -0.47 %
dirt-showdown            548   -2.68 %
dota2                   1747   -3.36 %
f1-2015                  776   -4.94 %
left_4_dead_2           1762   -0.07 %
metro_2033_redux        2670   -0.43 %
portal                   474   -0.22 %
talos_principle          324   -3.63 %
warsow                   176   -2.20 %
witcher2                1040  -73.78 %
----------------------------------------
All affected             991  -65.37 %  ... 9681 -> 3353
----------------------------------------
Total                  26725  -10.82 %  ... 58490 -> 52162

v2: treat Undef as both 0 and 1

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com> (v1)
Tested-by: Edmondo Tommasina <edmondo.tommasina@gmail.com> (v1)