nir: Add asserts to the casting functions

nir: Add asserts to the casting functions

This makes calling nir_foo_as_bar a bit safer because we're no longer 100%
trusting in the caller to ensure that it's safe.  The caller still needs to
do the right thing but this ensures that we catch invalid casts with an
assert rather than by reading garbage data.  The one downside is that we do
use the casts a bit in nir_validate and it's not a validate_assert.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>