[AMDGPU] Increase max iterations count to analyze complete unroll

Summary: In some cases inner loops may not get boosts so try to analyze them deeper.

Reviewers: rampitec, mzolotukhin

Reviewed By: rampitec

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, zzheng, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81204