<bit>: Remove tzcnt special case for small integers in countr_zero

[sylveon]

Fixes #1534

This moves the check for tzcnt inside the else case, improving codegen for small integer types (short and char)

Here's a Godbolt sample comparing the assembly output before and after: https://godbolt.org/z/K6nz8z

[StephanTLavavej]

Thanks for improving the performance of these functions, and for verifying the before-and-after codegen! 🚀 😸