[SCEV] Fix cache poisoning from depth-limited truncate expressions #850
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Don't insert depth-limited unsimplified SCEVTruncateExpr into the UniqueSCEVs cache. Caching these prevents later queries at lower depth from properly simplifying (e.g., trunc(AddRec) -> AddRec). This fixes an assertion failure in IndVarSimplify where getTruncateExpr on an AddRecExpr returned a cached SCEVTruncateExpr instead of the expected SCEVAddRecExpr.
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Don't insert depth-limited unsimplified SCEVTruncateExpr into the UniqueSCEVs cache. Caching these prevents later queries at lower depth from properly simplifying (e.g., trunc(AddRec) -> AddRec).
This fixes an assertion failure in IndVarSimplify where getTruncateExpr on an AddRecExpr returned a cached SCEVTruncateExpr instead of the expected SCEVAddRecExpr.