List of simplifications we should perform

[Richard Kenner]

    Unless, this simplification is just for if (!!j).
    With Andy's remarks, it makes sense to add another
    simplification to the list:

    if (j?1:0)  --> if (j)

    Can simplify-rtx do this ??

Actually, the tree-level simplifications do this, so it gets done even
at -O0.  If we force it to be done later (via an intermediate variable),
we see that this is *also* done by simplify-rtx, as is the case of
"if (!!j)" when split up.

GCC is very good at doing this sort of local simplification, so I'd be
surprised if any major ones were missing, but I will check.