AMDGPU: waitcnt operand fixes

Summary:
Allow lgkmcnt up to 0xF (hardware allows that).
Fix mask for ExpCnt in AMDGPUInstPrinter.

Reviewers: tstellarAMD, arsenm

Subscribers: arsenm

Differential Revision: http://reviews.llvm.org/D16314

Patch by: Nikolay Haustov

llvm-svn: 259059