[AMDGPU][Waitcnt] fix "comparison of integers of different signs" build error

Build error on Android; reported by and fix provided by (thanks) by Mauro Rossi <issor.oruam@gmail.com>

Fixes the following building error:

external/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp:1903:61:
error: comparison of integers of different signs:
'typename iterator_traits<__wrap_iter<MachineBasicBlock **> >::difference_type'
(aka 'int') and 'unsigned int' [-Werror,-Wsign-compare]
                      BlockWaitcntProcessedSet.end(), &MBB) < Count)) {
                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~
1 error generated.

Differential Revision: https://reviews.llvm.org/D49089

llvm-svn: 336588