Change RegexCompiler to special-case sets of just 2 or 3 chars

[stephentoub]

When we need to emit the code to match a character against a set, we call a helper that checks lots of conditions and determines the best code to output for that check, e.g. if the set is '\d', we just emit a call to char.IsDigit, or if the set contains a single range, we emit a simple range check. It turns out it's common to have sets with just two or three characters in them, in particular coming from our prefix checker that, e.g. for a global alternation, will generate a set containing the first possible characters of each branch. In master we generate a lookup table for such a set, but that involves computing the bit offset from the lookup table, doing the appropriate shifts, indexing, etc.; for two or three characters, it's cheaper to just perform the two or three comparison operations. For example, this makes a few percent improvement on regex redux, where we end up doing lots of these comparisons.

cc: @danmosemsft, @eerhardt, @ViktorHofer

[pgovind]

Ship it