Auto merge of #110837 - scottmcm:offset-for-add, r=compiler-errors

Use MIR's `Offset` for pointer `add` too

~~Status: draft while waiting for #110822 to land, since this is built atop that.~~
~~r? `@ghost~~`

Canonical Rust code has mostly moved to `add`/`sub` on pointers, which take `usize`, instead of `offset` which takes `isize`.  (And, relatedly, when `sub_ptr` was added it turned out it replaced every single in-tree use of `offset_from`, because `usize` is just so much more useful than `isize` in Rust.)

Unfortunately, `intrinsics::offset` could only accept `*const` and `isize`, so there's a *huge* amount of type conversions back and forth being done.  They're identity conversions in the backend, but still end up producing quite a lot of unhelpful MIR.

This PR changes `intrinsics::offset` to accept `*const` *and* `*mut` along with `isize` *and* `usize`.  Conveniently, the backends and CTFE already handle this, since MIR's `BinOp::Offset` [already supports all four combinations](adaac6b166/compiler/rustc_const_eval/src/transform/validate.rs (L523-L528)).

To demonstrate the difference, I added some `mir-opt/pre-codegen/` tests around slice indexing.  Here's the difference to `[T]::get_mut`, since it uses `<*mut _>::add` internally:
```diff
`@@` -79,30 +70,21 `@@` fn slice_get_mut_usize(_1: &mut [u32], _2: usize) -> Option<&mut u32> {
         StorageLive(_12);                // scope 3 at $SRC_DIR/core/src/slice/index.rs:LL:COL
         StorageLive(_9);                 // scope 6 at $SRC_DIR/core/src/slice/index.rs:LL:COL
         _9 = _8 as *mut u32 (PtrToPtr);  // scope 11 at $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
-        StorageLive(_13);                // scope 13 at $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
-        _13 = _2 as isize (IntToInt);    // scope 13 at $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
-        StorageLive(_14);                // scope 15 at $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
-        StorageLive(_15);                // scope 15 at $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
-        _15 = _9 as *const u32 (Pointer(MutToConstPointer)); // scope 15 at $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
-        _14 = Offset(move _15, _13);     // scope 15 at $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
-        StorageDead(_15);                // scope 15 at $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
-        _7 = move _14 as *mut u32 (PtrToPtr); // scope 15 at $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
-        StorageDead(_14);                // scope 15 at $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
-        StorageDead(_13);                // scope 13 at $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
+        _7 = Offset(_9, _2);             // scope 13 at $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
         StorageDead(_9);                 // scope 6 at $SRC_DIR/core/src/slice/index.rs:LL:COL
         StorageDead(_12);                // scope 3 at $SRC_DIR/core/src/slice/index.rs:LL:COL
         StorageDead(_11);                // scope 3 at $SRC_DIR/core/src/slice/index.rs:LL:COL
```
https://github.com/rust-lang/rust/pull/110837/commits/1c1c8e442add0f46905a57a25a6cba52b8b0c54d#diff-a841b6a4538657add3f39bc895744331453d0625e7aace128b1f604f0b63c8fdR80

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -819,8 +819,15 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     .builtin_deref(true)
                     .unwrap_or_else(|| bug!("deref of non-pointer {:?}", input_ty))
                     .ty;
-                let llty = bx.cx().backend_type(bx.cx().layout_of(pointee_type));
-                bx.inbounds_gep(llty, lhs, &[rhs])
+                let pointee_layout = bx.cx().layout_of(pointee_type);
+                if pointee_layout.is_zst() {
+                    // `Offset` works in terms of the size of pointee,
+                    // so offsetting a pointer to ZST is a noop.
+                    lhs
+                } else {
+                    let llty = bx.cx().backend_type(pointee_layout);
+                    bx.inbounds_gep(llty, lhs, &[rhs])
+                }
             }
             mir::BinOp::Shl => common::build_unchecked_lshift(bx, lhs, rhs),
             mir::BinOp::Shr => common::build_unchecked_rshift(bx, input_ty, lhs, rhs),