nir: add int_to_float lowering pass

nir: add int_to_float lowering pass

This new pass lowers ints and bools to floats. It allows hardware
that doesn't have native integers (e.g. Mali4x0) use the same
code paths as modern hardware.

It uses newly introduced pass to gather SSA types and should be
used as late as possible.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>