Add pointer masking convenience functions

This commit adds the following functions all of which have a signature
`pointer, usize -> pointer`:
- `<*mut T>::mask`
- `<*const T>::mask`
- `intrinsics::ptr_mask`

These functions are equivalent to `.map_addr(|a| a & mask)` but they
utilize `llvm.ptrmask` llvm intrinsic.

*masks your pointers*

compiler/rustc_typeck/src/check/intrinsic.rs

@@ -105,7 +105,8 @@ pub fn intrinsic_operation_unsafety(intrinsic: Symbol) -> hir::Unsafety {
         | sym::type_name
         | sym::forget
         | sym::black_box
-        | sym::variant_count => hir::Unsafety::Normal,
+        | sym::variant_count
+        | sym::ptr_mask => hir::Unsafety::Normal,
         _ => hir::Unsafety::Unsafe,
     }
 }
@@ -203,6 +204,15 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
                 ],
                 tcx.mk_ptr(ty::TypeAndMut { ty: param(0), mutbl: hir::Mutability::Not }),
             ),
+            sym::ptr_mask => (
+                1,
+                vec![
+                    tcx.mk_ptr(ty::TypeAndMut { ty: param(0), mutbl: hir::Mutability::Not }),
+                    tcx.types.usize,
+                ],
+                tcx.mk_ptr(ty::TypeAndMut { ty: param(0), mutbl: hir::Mutability::Not }),
+            ),
+
             sym::copy | sym::copy_nonoverlapping => (
                 1,
                 vec![