[ADT] Add all_equal predicate

`llvm::all_equal` checks if all values in the given range are equal, i.e., there are no two elements that are not equal.
Similar to `llvm::all_of`, it returns `true` when the range is empty.

`llvm::all_equal` is intended to supersede `llvm::is_splat`, which will be deprecated and removed in future patches.
See the discussion thread for more details:
https://discourse.llvm.org/t/adt-is-splat-and-empty-ranges/64692.

Reviewed By: dblaikie, shchenz

Differential Revision: https://reviews.llvm.org/D132334