CFI: add XFAIL test for a future optimization of two vcalls.

Summary:
Often, a code will call multiple virtual methods of a given object.
If they go in a linear block, it should be possible to check vtable
before the first call, then store vtable pointer and reuse it for
the second vcall without any additional checks.

This is expected to have a positive performance impact on a hot
path in Blink, see https://crbug.com/634139.

Reviewers: kcc

Differential Revision: https://reviews.llvm.org/D23151

llvm-svn: 277795