When matching an incompatible encoding, the Regexp needs to recompile.
If `usecnt == 0`, then we can reuse the `ptr` because nothing else is
using it. This avoids allocating another `regex_t`.

This speeds up matches that switch to incompatible encodings by 15%.

Branch:

```
Regex#match? with different encoding
                          1.431M (± 1.3%) i/s -      7.264M in   5.076153s
Regex#match? with same encoding
                         16.858M (± 1.1%) i/s -     85.347M in   5.063279s
```

Base:

```
Regex#match? with different encoding
                          1.248M (± 2.0%) i/s -      6.342M in   5.083151s
Regex#match? with same encoding
                         16.377M (± 1.1%) i/s -     82.519M in   5.039504s
```

Script:

```
regex = /foo/
str1 = "日本語"
str2 = "English".force_encoding("ASCII-8BIT")

Benchmark.ips do |x|
  x.report("Regex#match? with different encoding") do |times|
    i = 0
    while i < times
      regex.match?(str1)
      regex.match?(str2)
      i += 1
    end
  end

  x.report("Regex#match? with same encoding") do |times|
    i = 0
    while i < times
      regex.match?(str1)
      i += 1
    end
  end
end
```