[LoopInfo] Limit the iterations to check whether a loop has dedicated exits

for extreme large case.

We had a case that a single loop which has 4000 exits and the average number
of predecessors of each exit is > 1000, and we found compiling the case spent
a significant amount of time on checking whether a loop has dedicated exits.
This patch adds a limit for the iterations to the check. With the patch, the
time to compile our testcase reduced from 1000s to 200s (clang release build).

Differential Revision: https://reviews.llvm.org/D67359

llvm-svn: 372990