[SCEV] Make computeExitLimit more simple and more powerful

Current implementation of `computeExitLimit` has a big piece of code
the only purpose of which is to prove that after the execution of this
block the latch will be executed. What it currently checks is actually a
subset of situations where the exiting block dominates latch.

This patch replaces all these checks for simple particular cases with
domination check over loop's latch which is the only necessary condition
of taking the exiting block into consideration. This change allows to
calculate exact loop taken count for simple loops like

  for (int i = 0; i < 100; i++) {
    if (cond) {...} else {...}
    if (i > 50) break;
    . . .
  }

Differential Revision: https://reviews.llvm.org/D44677
Reviewed By: efriedma

llvm-svn: 329047