[Guards] Introduce loop-predication pass

This patch introduces guard based loop predication optimization. The new LoopPredication pass tries to convert loop variant range checks to loop invariant by widening checks across loop iterations. For example, it will convert

  for (i = 0; i < n; i++) {
    guard(i < len);
    ...
  }

to

  for (i = 0; i < n; i++) {
    guard(n - 1 < len);
    ...
  }

After this transformation the condition of the guard is loop invariant, so loop-unswitch can later unswitch the loop by this condition which basically predicates the loop by the widened condition:

  if (n - 1 < len)
    for (i = 0; i < n; i++) {
      ...
    } 
  else
    deoptimize

This patch relies on an NFC change to make ScalarEvolution::isMonotonicPredicate public (revision 293062).

Reviewed By: sanjoy

Differential Revision: https://reviews.llvm.org/D29034

llvm-svn: 293064