flatten nested slice patterns in HAIR construction

nested slice patterns have the same functionality as non-nested
ones, so flatten them in HAIR construction.

Fixes #26158.

src/test/compile-fail/issue-26158.rs

@@ -0,0 +1,16 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(slice_patterns)]
+
+fn main() {
+    let x: &[u32] = &[];
+    let &[[ref _a, ref _b..]..] = x; //~ ERROR refutable pattern
+}