Do not use void pointer for ptr_mask intrinsic

I couldn't find where exactly it's documented, but apperantly pointers to void
type are invalid in llvm - void is only allowed as a return type of functions.

src/test/codegen/intrinsics/mask.rs

@@ -4,8 +4,8 @@
 // CHECK-LABEL: @mask_ptr
 // CHECK-SAME: [[WORD:i[0-9]+]] %mask
 #[no_mangle]
-pub fn mask_ptr(ptr: *const u8, mask: usize) -> *const u8 {
+pub fn mask_ptr(ptr: *const u16, mask: usize) -> *const u16 {
     // CHECK: call
-    // CHECK-SAME: @llvm.ptrmask.p0isVoid.[[WORD]](
+    // CHECK-SAME: @llvm.ptrmask.p0i8.[[WORD]](
     core::intrinsics::ptr_mask(ptr, mask)
 }