remove const-support for align_offset

Operations like is_aligned would return actively wrong results at compile-time,
i.e. calling it on the same pointer at compiletime and runtime could yield
different results. That's no good.

Instead of having hacks to make align_offset kind-of work in const-eval, just
use const_eval_select in the few places where it makes sense, which also ensures
those places are all aware they need to make sure the fallback behavior is
consistent.

compiler/rustc_hir/src/lang_items.rs

@@ -348,9 +348,6 @@ language_item_table! {
 
     MaybeUninit,             sym::maybe_uninit,        maybe_uninit,               Target::Union,          GenericRequirement::None;
 
-    /// Align offset for stride != 1; must not panic.
-    AlignOffset,             sym::align_offset,        align_offset_fn,            Target::Fn,             GenericRequirement::None;
-
     Termination,             sym::termination,         termination,                Target::Trait,          GenericRequirement::None;
 
     Try,                     sym::Try,                 try_trait,                  Target::Trait,          GenericRequirement::None;