[mir-opt] Introduce a new flag to enable experimental/unsound mir opts

compiler/rustc_session/src/options.rs

@@ -1115,6 +1115,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         `hir,typed` (HIR with types for each node),
         `hir-tree` (dump the raw HIR),
         `mir` (the MIR), or `mir-cfg` (graphviz formatted MIR)"),
+    unsound_mir_opts: bool = (false, parse_bool, [TRACKED],
+        "enable unsound and buggy MIR optimizations (default: no)"),
     unstable_options: bool = (false, parse_bool, [UNTRACKED],
         "adds unstable command line options to rustc interface (default: no)"),
     use_ctors_section: Option<bool> = (None, parse_opt_bool, [TRACKED],