[llvm][FileCheck] Fix unit tests failures with EXPENSIVE_CHECKS

EXPENSIVE_CHECKS enables _GLIBCXX_DEBUG, which makes std::sort
check that the compare function is implemented correctly.

To do this it calls it with the first item as both sides.
Which trips the assert here because we think they're
2 capture ranges that overlap, when it's just the same range twice.

Check up front for the two sides being the same item
(same address, not just ==).

Reviewed By: kazu

Differential Revision: https://reviews.llvm.org/D130282