[NFC] Deduplicate comment in PromoteMemoryToRegister.cpp

This has been duplicated since before
2372a193, but that commit has it
appearing twice in the space of 10 lines of the same function body. It
could also be hoisted up to the point just after where the last
special-case is considered, but I want to keep the intent of the
original authors.

Committed as obvious without a review.