[tcb] Merge PHI nodes -- 7.4% reduction of PHI nodes (Take 2)

[Kazu Hirata]

Hi Jeff,

> BB3:
>   x1 = PHI (0 (BB0), 1 (BB1))
>   x2 = (_Bool) x1;              <- Notice this cast!
> 
> BB4:
>   x2 = PHI (x1 (BB3), x0 (BB2))
>   if (x2)
>     goto BB5;
>   else
>     goto BB6;

I meant

BB3:
  x1 = PHI (0 (BB0), 1 (BB1))
  x2 = (_Bool) x1;              <- Notice this cast!

BB4:
  x3 = PHI (x2 (BB3), x0 (BB2))
  if (x3)
    goto BB5;
  else
    goto BB6;

Kazu Hirata