[AMDGPU] Combine or remove redundant waitcnts at the end of each MBB (#87539)

Call generateWaitcnt unconditionally at the end of
SIInsertWaitcnts::insertWaitcntInBlock. Even if we don't need to
generate a new waitcnt instruction it has the effect of combining or
removing redundant waitcnts that were already present. Tests show
various small improvements in waitcnt placement.