Teach ScalarEvolution how to analyze loops with multiple exit

Teach ScalarEvolution how to analyze loops with multiple exit
blocks, and also exit blocks with multiple conditions (combined
with (bitwise) ands and ors). It's often infeasible to compute an
exact trip count in such cases, but a useful upper bound can often
be found.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73866 91177308-0d34-0410-b5e6-96231b3b80d8