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_codegen_llvm/src/context.rs

@@ -644,6 +644,7 @@ impl<'ll> CodegenCx<'ll, '_> {
 
         let i8p = self.type_i8p();
         let void = self.type_void();
+        let voidp = self.type_ptr_to(void);
         let i1 = self.type_i1();
         let t_i8 = self.type_i8();
         let t_i16 = self.type_i16();
@@ -886,6 +887,9 @@ impl<'ll> CodegenCx<'ll, '_> {
             ifn!("llvm.dbg.declare", fn(t_metadata, t_metadata) -> void);
             ifn!("llvm.dbg.value", fn(t_metadata, t_i64, t_metadata) -> void);
         }
+
+        ifn!("llvm.ptrmask", fn(voidp, t_isize) -> voidp);
+
         None
     }
 

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -71,6 +71,7 @@ fn get_simple_intrinsic<'ll>(
         sym::nearbyintf64 => "llvm.nearbyint.f64",
         sym::roundf32 => "llvm.round.f32",
         sym::roundf64 => "llvm.round.f64",
+        sym::ptr_mask => "llvm.ptrmask",
         _ => return None,
     };
     Some(cx.get_intrinsic(llvm_name))