bpo-39195: Check earlier if pattern is compiled in _compile

[Recursing]

Improve performance by checking for already compiled patterns before checking for the type of flags and accessing the module cache

https://bugs.python.org/issue39195

[the-knights-who-say-ni]

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@Recursing

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[Recursing]

I think I had already signed the agreement, signed it again anyway

Closing PR, see discussion on bpo