[PPC][LLVM] Inclusive terms: remove references to sanity check in lib/Target/PowerPC

Removed references to `sanity check` in `PPCBranchCoalescing.cpp` code comments.
No word substitution made in this case, as the comments and code following illustrated are
sufficient IMO.

Reviewed By: quinnp

Differential Revision: https://reviews.llvm.org/D112452