[InstCombine] Recognize another bswap idiom.

Summary:
The byte-swap recognizer can now notice that this

```
uint32_t bswap(uint32_t x)
{
  x = (x & 0x0000FFFF) << 16 | (x & 0xFFFF0000) >> 16;
  x = (x & 0x00FF00FF) << 8 | (x & 0xFF00FF00) >> 8;
  return x;
}
```
    
is a bswap. Fixes PR23863.

Reviewers: nlewycky, hfinkel, hans, jmolloy, rengolin

Subscribers: majnemer, rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D12637

llvm-svn: 248482