Removes the bool_to_float logic from the int_to_float pass, so that both
can be used separately. By having separate passes we have better validation
and it makes it possible to use with the lower_ftrunc option (int lowering
generates ftrunc, but lower_ftrunc generates bools, ftrunc lowering should
probably be reworked). For now we always expect lower_bool to come after
lower_int.

Also fixes f2i32 to become ftrunc and adds u2f/f2u cases.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>