Wrong size used for vectorization check in BitArray

[PranavSenthilnathan]

Vector512 should be Vector128:

runtime/src/libraries/System.Collections/src/System/Collections/BitArray.cs

Line 888 in a73f163

else if (Ssse3.IsSupported && ((uint)m_length >= Vector512<byte>.Count * 2u))

Found by @saucecontrol

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[stephentoub]

This ends up only being a performance bug, not functional / reliability, correct? We'll end up falling back to the scalar path more often than we otherwise could?

[saucecontrol]

Yep, that's all. I suggested it as a good first PR on the community discord.