Optimize _validate(_, as:) for short buffers (#92505)
Speeds up `String(validating:, as: ASCII.self)` by ~3.5x at N=4, 1.24x
at N=256. Similar but less dramatic (2.5x - 1.04x) for UTF8 and UTF16,
and even less dramatic (1.4x - 1.02x) for `String(decoding:, as:
UTF16.self)`.
Fixes rdar://188752366
