[patch, RFC] Fix PR23821: DOM and VRP creating harder to optimize code

[Sebastian Pop]

Hi,

Symbolic propagation is causing problems downstream in the loop
optimizers.  In PR23821, this is because both VRP and DOM would
produce a code that is hard to analyze.  This patch analyzes the code
before and after a symbolic propagation, and discards those
substitutions that degrade the accuracy of the analysis.

I will wait for comments on the approach before sending this patch
to regstrap.

-- 
Sebastian
AMD - GNU Tools
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