Auto merge of #125025 - saethlin:clone-your-body, r=<try>

[perf experiments] Clone all MIR bodies

We keep saying things like cloning all the MIR would be expensive, but... how expensive actually?

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -166,6 +166,8 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     let llfn = cx.get_fn(instance);
 
     let mir = cx.tcx().instance_mir(instance.def);
+    let body: rustc_middle::mir::Body<'tcx> = mir.clone();
+    drop(std::hint::black_box(body));
 
     let fn_abi = cx.fn_abi_of_instance(instance, ty::List::empty());
     debug!("fn_abi: {:?}", fn_abi);