Redundant logical operations left after early splitting

[Jeff Law]

hutchinsonandy@aim.com wrote:
> I still have to try extra fwprop pass to confirm this solves the issue.
> 
> But it does seem that the missing piece is effective constant 
> propagation + simplification after splits and subregs - which is 
> currently ineffective in local-alloc (or any later pass)
> 
> Adding extra pass indeed seems a poor route - BUT this would suggest no 
> need for any constant propagation in local-alloc. Thus avoid duplication 
> of function.
We need to look at the tradeoffs involved -- adding more passes is
always possible, but that comes with a cost and it's unclear how many
ports are going to benefit and how big those benefits might be.

You're going to see this kind of problem raise its ugly head anytime
a port chooses keep operations at a higher level early in the RTL
optimizers, then lower them to their component operations in the
split_all_insns pass.

Jeff