[committed] operator_abs::fold_range() returning incorrect result for overflows (pr92506)

[Andrew MacLeod]

Traced it back to a typo in operator_abs::fold_range() when I did the 
conversion where the wrong line got copied in..

Instead of returning value_range (type) when a overflow happens, it was 
returning the same result of the previous check, which was the case for 
all positives.
This had EVRP setting the range of an ABS to [-MIN, -1] instead of 
varying, which later caused VRP to intersect that with 0 - [-MIN, -1] 
and all heck broke loose. doh.

I also stumbled across a case where we should be starting with undefined 
in the default fold_range() and building with union for each sub-range.  
We previously declared a local value_range to work with, and that 
defaulted to undefined.  When I changed it to a reference parameter, I 
need to explicitly initialize it.

Bootstraps, checked in as revision 278259.

Andrew
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