[Polly] Add XFAIL to large-numbers-in-boundary-context.ll

After rL315683 (improve SCEV to calculate max BETakenCount when end
bound of loop is variant and loop is of form {Start,+1, Stride} LT End)
this test in polly started failing.
However, as discussed in https://reviews.llvm.org/rL315683,
this polly test is not a loops bound test and the MaxBECount calculated by
SCEV looks correct. The max BECount is the value calculated even when the end
bound of loop is invariant.

As discussed with Tobias offline, I'm marking this as an XFAIL, until he
gets a chance to update the testcase, so the build bot goes to green.

llvm-svn: 315912