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

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.

Differential Revision: https://reviews.llvm.org/D29505

llvm-svn: 294032