[AMDGPU][Waitcnt] Fix handling of loops with many bottom blocks

In terms of waitcnt insertion/if necessary, the waitcnt pass forces convergence
for a loop. Previously, that kicked if greater than 2 passes over a loop, which
doesn't account for loop with many bottom blocks. So, increase the threshold to
(n+1), where n is the number of bottom blocks. This gives the pass an
opportunity to consider the contribution of each bottom block, to the overall
loop, before the forced convergence potentially kicks in.

Differential Revision: https://reviews.llvm.org/D47488

llvm-svn: 333556