Auto merge of #82884 - nagisa:nagisa/remove-most-of-sideeffect-inserts, r=nikic

Remove the -Zinsert-sideeffect

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.