[LoopUnroll][NFC] Simplify recent block frequency tests (#177025)

Refactor a number of recent tests in
`llvm/test/Transforms/LoopUnroll/branch-weights-freq` to make it easier
to understand and extend them.

The changes mostly resemble the refactoring I recently did in PR #165635
in response to reviewer comments:
- For each case (e.g., each `-unroll-count` value in
`unroll-epilog.ll`), group all FileCheck directives together. That way,
while digesting a single case, the reader does not need to sift through
all other cases and a complex FileCheck prefix scheme.
- Reduce CFG testing. Drop many FileCheck directives that check for all
basic block labels and branches, and drop the cryptic
`-implicit-check-not` that excludes others. Instead, just use positive
checks for every loop body (represented by `call void @f`), for relevant
metadata, and for the branch instructions to which the metadata is
attached, and use simple negative checks (e.g.,
`-implicit-check-not='!prof'`) to be sure we have not missed any.
- Better document what the test intends to cover.

The result is sometimes longer tests due to comments and repetition, but
I believe they are easier to maintain this way.

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