Auto merge of #98112 - saethlin:mir-alignment-checks, r=oli-obk

Insert alignment checks for pointer dereferences when debug assertions are enabled

Closes rust-lang/rust#54915

- [x] Jake tells me this sounds like a place to use `MirPatch`, but I can't figure out how to insert a new basic block with a new terminator in the middle of an existing basic block, using `MirPatch`. (if nobody else backs up this point I'm checking this as "not actually a good idea" because the code looks pretty clean to me after rearranging it a bit)
- [x] Using `CastKind::PointerExposeAddress` is definitely wrong, we don't want to expose. Calling a function to get the pointer address seems quite excessive. ~I'll see if I can add a new `CastKind`.~ `CastKind::Transmute` to the rescue!
- [x] Implement a more helpful panic message like slice bounds checking.

r? `@oli-obk`

Author: bors

src/base.rs

@@ -379,6 +379,18 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
                             source_info.span,
                         );
                     }
+                    AssertKind::MisalignedPointerDereference { ref required, ref found } => {
+                        let required = codegen_operand(fx, required).load_scalar(fx);
+                        let found = codegen_operand(fx, found).load_scalar(fx);
+                        let location = fx.get_caller_location(source_info).load_scalar(fx);
+
+                        codegen_panic_inner(
+                            fx,
+                            rustc_hir::LangItem::PanicBoundsCheck,
+                            &[required, found, location],
+                            source_info.span,
+                        );
+                    }
                     _ => {
                         let msg_str = msg.description();
                         codegen_panic(fx, msg_str, source_info);