radeonsi: set IF_THRESHOLD to 4

radeonsi: set IF_THRESHOLD to 4

In 74e39de9324d it was set to 3 and it was reported that 4 caused
tesseract to start spilling VGPRs. This no longer seems to be the
case.

Totals:
SGPRS: 2787844 -> 2787764 (-0.00 %)
VGPRS: 1713121 -> 1712717 (-0.02 %)
Spilled SGPRs: 7532 -> 7532 (0.00 %)
Spilled VGPRs: 49 -> 33 (-32.65 %)
Private memory VGPRs: 2060 -> 2060 (0.00 %)
Scratch size: 2200 -> 2180 (-0.91 %) dwords per thread
Code Size: 79265520 -> 79248360 (-0.02 %) bytes
LDS: 436 -> 436 (0.00 %) blocks
Max Waves: 670535 -> 670608 (0.01 %)
Wait states: 0 -> 0 (0.00 %)

Before:
 VGPR SPILLING APPS   Shaders SpillVGPR  PrivVGPR ScratchSize
 EffectsCaveDemo          301         0       256       264
 ReflectionsSubwayDemo    264         0       256       264
 VehicleGame              295         0       128       132
 bioshock-infinite       1140         0       448       516
 dirt-showdown            453        33         0        28
 gang-beasts              364         0       500       496
 kerbal-space-program    1228         0       472       480
 tomb-raider-ultra       1199        16         0        20

After:
 VGPR SPILLING APPS   Shaders SpillVGPR  PrivVGPR ScratchSize
 EffectsCaveDemo          301         0       256       264
 ReflectionsSubwayDemo    264         0       256       264
 VehicleGame              295         0       128       132
 bioshock-infinite       1140         0       448       516
 dirt-showdown            453        33         0        28
 gang-beasts              364         0       500       496
 kerbal-space-program    1228         0       472       480

The only change in VGPR spills is the elimination of all spills
in Tomb Raider at Ultra settings. Closer examination shows that
the shaders go over the limit because they contain three
expressions a mul, rcp and ubo load. The ubo load is actually
used elsewhere and is therefore stored in a temp already in IR
such as tgsi but glsl ir counts it agaist the if cost.

Acked-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Acked-by: Marek Olšák <marek.olsak@amd.com>