Draft: Use foldl' in OccurAnal.

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• Swap out foldr in Core.Opt.OccurAnal for foldl'. The idea is that foldl' can be tail call optimized while foldr can't so in cases where short circuiting behavior is not required, we can get some minor speedups with foldl'.

MR related to !6761

I'm marking as draft, if the numbers look good then its an easy change to proliferate to other modules.