[LoopDist] Only run LAA for loops with the pragma

This should fix some compile-time regressions after r267672.  Thanks to
Chris Matthews for bisecting it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@269392 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed