Auto merge of #53080 - hermord:rc-opt, r=alexcrichton

Auto merge of #53080 - hermord:rc-opt, r=alexcrichton

Change `Rc::inc_{weak,strong}` to better hint optimization to LLVM

As discussed in #13018, `Rc::inc_strong` and `Rc::inc_weak` are changed to allow compositions of `clone` and `drop` to be better optimized. Almost entirely as in [this comment](https://github.com/rust-lang/rust/issues/13018#issuecomment-408642184), except that `abort` on zero is added so that a `drop(t.clone())` does not produce a zero check followed by conditional deallocation.

This is different from #21418 in that it doesn't rely on `assume`, avoiding the prohibitive compilation slowdown.

[Before and after IR](https://gist.github.com/hermord/266e55451b7fe0bb8caa6e35d17c86e1).