Rollup merge of #95831 - redzic:xor-uppercase, r=workingjubilee

Rollup merge of #95831 - redzic:xor-uppercase, r=workingjubilee

Use bitwise XOR in to_ascii_uppercase

This saves an instruction compared to the previous approach, which
was to unset the fifth bit with bitwise OR.

Comparison of generated assembly on x86: https://godbolt.org/z/GdfvdGs39

This can also affect autovectorization, saving SIMD instructions as well: https://godbolt.org/z/cnPcz75T9

Not sure if `u8::to_ascii_lowercase` should also be changed, since using bitwise OR for that function does not require an extra bitwise negate since the code is setting a bit rather than unsetting a bit. `char::to_ascii_uppercase` already uses XOR, so no change seems to be required there.