make Size::from* methods generic in the integer type they accept

src/librustc_codegen_ssa/mir/operand.rs

@@ -91,7 +91,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                 };
                 let a = Scalar::from(Pointer::new(
                     bx.tcx().alloc_map.lock().create_memory_alloc(data),
-                    Size::from_bytes(start as u64),
+                    Size::from_bytes(start),
                 ));
                 let a_llval = bx.scalar_to_backend(
                     a,