Auto merge of rust-lang#96946 - WaffleLapkin:ptr_mask, r=scottmcm

Add pointer masking convenience functions

This PR adds the following public API:
```rust
impl<T: ?Sized> *const T {
    fn mask(self, mask: usize) -> *const T;
}

impl<T: ?Sized> *mut T {
    fn mask(self, mask: usize) -> *const T;
}

// mod intrinsics
fn mask<T>(ptr: *const T, mask: usize) -> *const T
```
This is equivalent to `ptr.map_addr(|a| a & mask)` but also uses a cool llvm intrinsic.

Proposed in rust-lang#95643 (comment)

cc `@Gankra` `@scottmcm` `@RalfJung`

r? rust-lang/libs-api

src/intrinsics/mod.rs

@@ -577,6 +577,13 @@ fn codegen_regular_intrinsic_call<'tcx>(
             ret.write_cvalue(fx, CValue::by_val(res, base.layout()));
         }
 
+        sym::ptr_mask => {
+            intrinsic_args!(fx, args => (ptr, mask); intrinsic);
+            let ptr = ptr.load_scalar(fx);
+            let mask = mask.load_scalar(fx);
+            fx.bcx.ins().band(ptr, mask);
+        }
+
         sym::transmute => {
             intrinsic_args!(fx, args => (from); intrinsic);