RFC: Remove REG_WAS_0.

[Kazu Hirata]

Hi Stephane,

> Instead of having such REG_WAS_ note, I would have liked to have
> access to a rtx giving the value of each register.  Similar to what
> we do in cse reg where we keep up the last rtx assigned to each reg.

The part of m68hc11.c that uses REG_WAS_0 tries to achieve the same
effect as move2add, except that you do so on registers that satisfy
H_REG_P only as you point out in

http://gcc.gnu.org/ml/gcc-patches/2003-04/msg00057.html

I'll do something so you can cleanly use move2add, which should be
better because even if REG_WAS_0 worked as you expected, you could
only go from (const_int 0) to either (const_int 1) or (const_int -1),
whereas with move2add, you can go from (const_int N) to (const_int N +
1) or (const_int N - 1) for any integer N using inc, inx, and iny
instructions.  Correct?

Kazu Hirata