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*

src/intrinsics/mod.rs

@@ -540,6 +540,13 @@ 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_val = ptr.load_scalar(fx);
+
+            fx.bcx.ins().band(ptr_val, mask);
+        }
+
         sym::transmute => {
             intrinsic_args!(fx, args => (from); intrinsic);