Auto merge of #111082 - saethlin:box-assertkind, r=saethlin

Box AssertKind

r? `@nnethercote` this feels like your kind of thing

I want to add a new variant to `AssertKind` that needs 3 operands, and that ends up breaking a bunch of size assertions. So... what if we go the opposite direction first; shrinking `AssertKind` by boxing it?

compiler/rustc_mir_build/src/build/scope.rs

@@ -1172,7 +1172,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             TerminatorKind::Assert {
                 cond,
                 expected,
-                msg,
+                msg: Box::new(msg),
                 target: success_block,
                 unwind: UnwindAction::Continue,
             },