[AMDGPU] Bump -amdgpu-unroll-threshold-private to 2000

[rampitec]

Summary

This has quite positive performance impact according to measurements.
Before previous fixes to limit the optimization that was too high
and blowed compile time and scratch usage, but now this is gone and
we can bump the threshold.

llvm/trunk/lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp

 #include "llvm/Target/TargetLowering.h"
 using namespace llvm;
 
 #define DEBUG_TYPE "AMDGPUtti"
 
 static cl::opt<unsigned> UnrollThresholdPrivate(
   "amdgpu-unroll-threshold-private",
   cl::desc("Unroll threshold for AMDGPU if private memory used in a loop"),
-  cl::init(800), cl::Hidden);
+  cl::init(2000), cl::Hidden);
 
 void AMDGPUTTIImpl::getUnrollingPreferences(Loop *L,
                                             TTI::UnrollingPreferences &UP) {
   UP.Threshold = 300; // Twice the default.
   UP.MaxCount = UINT_MAX;
   UP.Partial = true;
 
   // TODO: Do we want runtime unrolling?