radeonsi: set IF_THRESHOLD to 3

Piglit regressions (radeonsi or LLVM bugs, they pass on softpipe):
- glsl-1.10/execution/variable-indexing/vs-output-array-vec3-index-wr
- glsl-1.10/execution/variable-indexing/vs-output-array-vec4-index-wr
- glsl-110/execution/variable-indexing/vs-temp-array-mat2-index-col-row-wr
- glsl-110/execution/variable-indexing/vs-temp-array-mat2-index-row-wr

Totals:
SGPRS: 1132185 -> 1168801 (3.23 %)
VGPRS: 907856 -> 906204 (-0.18 %)
Spilled SGPRs: 2011 -> 2425 (20.59 %)
Spilled VGPRs: 368 -> 96 (-73.91 %)
Scratch VGPRs: 1344 -> 1060 (-21.13 %) dwords per thread
Code Size: 35916164 -> 35705372 (-0.59 %) bytes
LDS: 767 -> 767 (0.00 %) blocks
Max Waves: 194010 -> 194921 (0.47 %)
Wait states: 0 -> 0 (0.00 %)

Before:
 VGPR SPILLING APPS   Shaders SpillVGPR ScratchVGPR
 alien_isolation         2938        38        40
 bioshock-infinite       1769       245       732
 dirt-showdown            548        85        72
 f1-2015                  776         0       320
 ue4_lightroom_inter..     74         0       180

After:
 VGPR SPILLING APPS   Shaders SpillVGPR ScratchVGPR
 alien_isolation         2938        38        40
 bioshock-infinite       1769         0       480
 dirt-showdown            548        58        40
 f1-2015                  776         0       320
 ue4_lightroom_inter..     74         0       180

Bioshock and DiRT benefit.

If I set IF_THRESHOLD=4, tesseract starts spilling VGPRs

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>

src/gallium/drivers/radeonsi/si_pipe.c

@@ -632,6 +632,8 @@ static int si_get_shader_param(struct pipe_screen* pscreen, unsigned shader, enu
 		return 32;
 	case PIPE_SHADER_CAP_PREFERRED_IR:
 		return PIPE_SHADER_IR_TGSI;
+	case PIPE_SHADER_CAP_LOWER_IF_THRESHOLD:
+		return 3;
 
 	/* Supported boolean features. */
 	case PIPE_SHADER_CAP_TGSI_CONT_SUPPORTED:
@@ -659,7 +661,6 @@ static int si_get_shader_param(struct pipe_screen* pscreen, unsigned shader, enu
 	case PIPE_SHADER_CAP_SUPPORTED_IRS:
 	case PIPE_SHADER_CAP_TGSI_DROUND_SUPPORTED:
 	case PIPE_SHADER_CAP_TGSI_DFRACEXP_DLDEXP_SUPPORTED:
-	case PIPE_SHADER_CAP_LOWER_IF_THRESHOLD:
 		return 0;
 	}
 	return 0;