use is_multiple_of and div_ceil

[folkertdev]

In tricky logic, these functions are much more informative than the manual implementations. They also catch subtle bugs:

• the manual is_multiple_of often does not handle division by zero
• manual div_ceil often does not consider overflow

The transformation is free for is_multiple_of if the divisor is compile-time known to be non-zero. For div_ceil there is a small cost to considering overflow. Here is some assembly https://godbolt.org/z/5zP8KaE1d.

[rustbot]

r? @lcnr

rustbot has assigned @lcnr.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[rustbot]

Portable SIMD is developed in its own repository. If possible, consider making this change to rust-lang/portable-simd instead.

cc @calebzulawski, @programmerjake

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri