[SCEV] Do not strengthen nuw/nsw flags during get[Zero,Sign]ExtendedExpr.

Modifying AddRecs when constructing other expressions can lead to
surprising changes. It also seems like it is not really beneficial i
most cases.

At the moment, there's a single regression, but we still might be able
to improve the flags at AddRec construction.

Might help with the issue discussed in D143409.

Reviewed By: mkazantsev

Differential Revision: https://reviews.llvm.org/D144051