lld: Slightly simplify code and add comment.
Cherry-pick lld r315658 by Rui Ueyama:
This is not a mechanical transformation. Even though I believe this patch is correct, I'm not 100% sure if lld with this patch behaves exactly the same way as before on all edge cases. At least all tests still pass. I'm submitting this patch because it took almost a day to understand this function, and I don't want to lose it.
This fixes jemalloc assertion failures observed at startup with i386
binaries and an lld-linked libc.so.
Reviewed by: dim
Obtained from: LLVM r315658
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D13503