Not all bit-sizes may be supported natively in hardware for all operations.
This pass allows drivers to lower such operations to a bit-size that is
actually supported and then converts the result back to the original
bit-size.

Compiler backends control which operations and wich bit-sizes require
the lowering through a callback function.

v2: generalize this pass and make it available in NIR core (Rob, Jason)
v3: remove some temporaries and reduce nesting in instruction loop using
    a continue statement (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>