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

Insert alignment checks for pointer dereferences when debug assertions are enabled

Closes https://github.com/rust-lang/rust/issues/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`

compiler/rustc_mir_transform/src/lib.rs

@@ -90,6 +90,7 @@ mod separate_const_switch;
 mod shim;
 mod ssa;
 // This pass is public to allow external drivers to perform MIR cleanup
+mod check_alignment;
 pub mod simplify;
 mod simplify_branches;
 mod simplify_comparison_integral;
@@ -545,6 +546,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         tcx,
         body,
         &[
+            &check_alignment::CheckAlignment,
             &reveal_all::RevealAll, // has to be done before inlining, since inlined code is in RevealAll mode.
             &lower_slice_len::LowerSliceLenCalls, // has to be done before inlining, otherwise actual call will be almost always inlined. Also simple, so can just do first
             &unreachable_prop::UnreachablePropagation,