[LoopUnroll] Respect the convergent attribute.

Summary:
Specifically, when we perform runtime loop unrolling of a loop that
contains a convergent op, we can only unroll k times, where k divides
the loop trip multiple.

Without this change, we'll happily unroll e.g. the following loop

  for (int i = 0; i < N; ++i) {
    if (i == 0) convergent_op();
    foo();
  }

into

  int i = 0;
  if (N % 2 == 1) {
    convergent_op();
    foo();
    ++i;
  }
  for (; i < N - 1; i += 2) {
    if (i == 0) convergent_op();
    foo();
    foo();
  }.

This is unsafe, because we've just added a control-flow dependency to
the convergent op in the prelude.

In general, runtime unrolling loops that contain convergent ops is safe
only if we don't have emit a prelude, which occurs when the unroll count
divides the trip multiple.

Reviewers: resistor

Subscribers: llvm-commits, mzolotukhin

Differential Revision: http://reviews.llvm.org/D17526

llvm-svn: 263509