Vectorize basic_string::find (the string needle overload)

[AlexGuteniev]

Towards #5036

Benchmark	main	this
member_find<std::string>/0	1149 ns	292 ns
member_find<std::string>/1	1174 ns	300 ns
member_find<std::string>/2	52.1 ns	31.5 ns
member_find<std::string>/3	30.4 ns	18.1 ns
member_find<std::string>/4	14420 ns	1395 ns
member_find<std::string>/5	22193 ns	11719 ns
member_find<std::wstring>/0	943 ns	502 ns
member_find<std::wstring>/1	1087 ns	594 ns
member_find<std::wstring>/2	81.5 ns	58.6 ns
member_find<std::wstring>/3	42.5 ns	28.6 ns
member_find<std::wstring>/4	6463 ns	7656 ns
member_find<std::wstring>/5	18725 ns	21484 ns

[StephanTLavavej]

Thanks! 🧶 I pushed a minor stylistic change and a syntax fix.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for extending vectorization to this important member function! 🚀 ✨ 💚