[RuntimeUnroll] Add heuristic for unrolling multi-exit loop

Add a profitability heuristic to enable runtime unrolling of multi-exit
loop: There can be atmost two unique exit blocks for the loop and the
second exit block should be a deoptimizing block. Also, there can be one
other exiting block other than the latch exiting block. The reason for
the latter is so that we limit the number of branches in the unrolled
code to being at most the unroll factor.  Deoptimizing blocks are rarely
taken so these additional number of branches created due to the
unrolling are predictable, since one of their target is the deopt block.

Reviewers: apilipenko, reames, evstupac, mkuper

Subscribers: llvm-commits

Reviewed by: reames

Differential Revision: https://reviews.llvm.org/D35380

llvm-svn: 313363