[LoopUnroll][NFCI] Clean up remainder followup metadata handling (#165272)

Followup metadata for remainder loops is handled by two implementations,
both added by 7244852557ca6:

1. `tryToUnrollLoop` in `LoopUnrollPass.cpp`.
2. `CloneLoopBlocks` in `LoopUnrollRuntime.cpp`.

As far as I can tell, 2 is useless: I added `assert(!NewLoopID)` for the
`NewLoopID` returned by the `makeFollowupLoopID` call, and it never
fails throughout check-all for my build.

Moreover, if 2 were useful, it appears it would have a bug caused by
7cd826a321d9. That commit skips adding loop metadata to a new remainder
loop if the remainder loop itself is to be completely unrolled because
it will then no longer be a loop. However, that commit incorrectly
assumes that `UnrollRemainder` dictates complete unrolling of a
remainder loop, and thus it skips adding loop metadata even if the
remainder loop will be only partially unrolled.

To avoid further confusion here, this patch removes 2. check-all
continues to pass for my build. If 2 actually is useful, please advise
so we can create a test that covers that usage.

Near 2, this patch retains the `UnrollRemainder` guard on the
`setLoopAlreadyUnrolled` call, which adds `llvm.loop.unroll.disable` to
the remainder loop. That behavior exists both before and after
7cd826a321d9. The logic appears to be that remainder loop unrolling
(whether complete or partial) is opt-in. That is, unless
`UnrollRemainder` is true, `UnrollRuntimeLoopRemainder` skips running
remainder loop unrolling, and `llvm.loop.unroll.disable` suppresses any
later attempt at it.

This patch also extends testing of remainder loop followup metadata to
be sure remainder loop partial unrolling is handled correctly by 1.