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simplifier: Zap Id unfoldings before constructing InScopeSet in simpleOptExpr

Matthew Pickering requested to merge wip/fix-in-scope-simple-opt-expr into master

Care must be taken to remove unfoldings from Vars collected by exprFreeVars before using them to construct an in-scope set. Consider calling simpleOptExpr on an expression like

 case x of (a,b) -> (x,a)
  • One of those two occurrences of x has an unfolding (the one in (x,a), with unfolding x = (a,b)) and the other does not. (Inside a case GHC adds unfolding-info to the scrutinee's Id.)
  • But exprFreeVars just builds a set, so it's a bit random which occurrence is collected.
  • Then simpleOptExpr replaces each occurrence of x with the one in the in-scope set.
  • ! Bad bad bad: then the x in case x of ... may be replaced with a version that has an unfolding.

Fixes #25790 (closed)

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