Rollup merge of #100208 - RalfJung:dyn-upcast-nop, r=petrochenkov

make NOP dyn casts not require anything about the vtable

As suggested [on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-types/topic/dyn-upcasting.20stabilization/near/292151439). This matches what the codegen backends already do, and what Miri did do until https://github.com/rust-lang/rust/pull/99420 when I made it super extra paranoid.

compiler/rustc_codegen_ssa/src/base.rs

@@ -151,6 +151,7 @@ pub fn unsized_info<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
             let old_info =
                 old_info.expect("unsized_info: missing old info for trait upcasting coercion");
             if data_a.principal_def_id() == data_b.principal_def_id() {
+                // A NOP cast that doesn't actually change anything, should be allowed even with invalid vtables.
                 return old_info;
             }