This removes all of the code we had in place to work-around LLVM's
handling of forward progress. From this removal excluded is a workaround
where we'd insert a `sideeffect` into clearly infinite loops such as
`loop {}`. This code remains conditionally effective when the LLVM
version is earlier than 12.0, which fixed the forward progress related
miscompilations at their root.
LLVM assumes that a thread will eventually cause side effect. This is
not true in Rust if a loop or recursion does nothing in its body,
causing undefined behavior even in common cases like `loop {}`.
Inserting llvm.sideeffect fixes the undefined behavior.
As a micro-optimization, only insert llvm.sideeffect when jumping back
in blocks or calling a function.
A patch for LLVM is expected to allow empty non-terminate code by
default and fix this issue from LLVM side.
https://github.com/rust-lang/rust/issues/28728
