Lower intrinsics::offset to mir::BinOp::Offset

They're semantically the same, so this means the backends don't need to handle the intrinsic and means fewer MIR basic blocks in pointer arithmetic code.

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -559,11 +559,20 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
     }
 
     fn binary_ptr_op(
-        _ecx: &InterpCx<'mir, 'tcx, Self>,
-        _bin_op: mir::BinOp,
-        _left: &ImmTy<'tcx>,
-        _right: &ImmTy<'tcx>,
+        ecx: &InterpCx<'mir, 'tcx, Self>,
+        bin_op: mir::BinOp,
+        left: &ImmTy<'tcx>,
+        right: &ImmTy<'tcx>,
     ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
+        if bin_op == mir::BinOp::Offset {
+            let ptr = left.to_scalar().to_pointer(ecx)?;
+            let offset_count = right.to_scalar().to_target_isize(ecx)?;
+            let pointee_ty = left.layout.ty.builtin_deref(true).unwrap().ty;
+
+            let offset_ptr = ecx.ptr_offset_inbounds(ptr, pointee_ty, offset_count)?;
+            return Ok((Scalar::from_maybe_pointer(offset_ptr, ecx), false, left.layout.ty));
+        }
+
         throw_unsup_format!("pointer arithmetic or comparison is not supported at compile-time");
     }