[patch] Fix some pessimizations due to fixes for PRs 27144 and 27639

[Roger Sayle]

On Mon, 19 Jun 2006, Zdenek Dvorak wrote:
>
> 	* tree-ssa-loop-niter.c (implies_ge_p): New function.
> 	(derive_constant_upper_bound): Handle OP0 - CST in unsigned types
> 	correctly.

If Daniel has no objections, this is OK for mainline.  However, I
think we should try to restructure/rationalize the implies_ge_p logic
at some point in the future.  This form of logical inference testing
is useful at several points in the compiler, including DOM and fold.

Thanks for fixing this regression so quickly.

Roger
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