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.

compiler/rustc_interface/src/tests.rs

@@ -560,7 +560,6 @@ fn test_debugging_options_tracking_hash() {
     tracked!(inline_mir, Some(true));
     tracked!(inline_mir_threshold, Some(123));
     tracked!(inline_mir_hint_threshold, Some(123));
-    tracked!(insert_sideeffect, true);
     tracked!(instrument_coverage, true);
     tracked!(instrument_mcount, true);
     tracked!(link_only, true);