Use f{32,64}::to_bits for is_zero test in vec::SpecFromElem

vec::SpecFromElem provides an optimization to use calloc to fill a Vec
when the element given to fill the Vec is represented by 0.

For floats, the test for that currently used is `x == 0. &&
x.is_sign_positive()`. When compiled in a standalone function, rustc
generates the following assembly:

```
  xorps xmm1, xmm1
  ucomisd xmm0, xmm1
  setnp al
  sete cl
  and cl, al
  movq rax, xmm0
  test rax, rax
  setns al
  and al, cl
  ret
```

A simpler test telling us whether the value is represented by 0, is
`x.to_bits() == 0`, which rustc compiles to:

```
  movq rax, xmm0
  test rax, rax
  sete al
  ret
```

Not that the test is hot in any way, but it also makes it clearer what
the intent in the rust code is.

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