[Analyzer]: Regex.IsMatch guarding Regex.Match

[stephentoub]

In looking through various code bases, I've seen the following pattern repeatedly:

if (Regex.IsMatch(pattern))
{
    Match m = Regex.Match(pattern);
    ...
}

That just doubles the work involved when there is a match. It should instead be:

if (Regex.Match(pattern) is { Success: true } m)
{
    ...
}

or something similar. We should have an analyzer that flags these duplicative guards and a fixer that transforms into something like the latter.

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