SIMD comparison operators are always signed.

The compiler always emits signed SIMD comparison instructions, even for unsigned element types:

``` rust
use std::simd::u32x4;
u32x4(1, 2, 3, 1 << 31) < u32x4(2, 3, 4, 0)
// => u32x4(4294967295, 4294967295, 4294967295, 4294967295)
// i.e. <true, true, true, true>. expected <true, true, true, false>
```
