[LoopReroll] Remove unused and unmaintained pass (#80972)

Remove the LoopReroll pass, which is both unused (in any default
pipeline) and unmaintained, with numerous open correctness issues
(https://github.com/llvm/llvm-project/issues?q=is%3Aissue+is%3Aopen+loop-reroll).
The removal is in line with
https://discourse.llvm.org/t/rfc-disallow-unmaintained-unused-passes/75151.

There is also a defunct `-freroll-loops` option in clang, which I'll
remove separately.

Migrated from https://reviews.llvm.org/D150684.