[patch] alias.c: Replace "return NULL_RTX" with "return 0".

[Kazu Hirata]

Hi Fergus,

> The use of 0 is not universal -- a `grep -w' for NULL returns nearly
> 4,000 hits.  Adding in all the occurrences of NULL_RTX would bring it
> to about 5,500.  Admittedly `grep -w' for 0 returns nearly 10 times as
> many hits, but very many of those will be for integers rather than pointers.

The numbers sound a bit daunting, but IMHO we should settle on one
way, whether that is 0 or NULL_RTX.  (I am willing to work on the
actual conversion process.)  Having many ways of saying one thing is
confusing.

Kazu Hirata