Bugfix: SCEV incorrectly marks certain expressions as nsw

I could not come up with a test case for this one; but I don't think
`getPreStartForSignExtend` can assume `AR` is `nsw` -- there is one
place in scalar evolution that calls `getSignExtendAddRecStart(AR,
...)` without proving that `AR` is `nsw`

(line 1564)

   OperandExtendedAdd =
     getAddExpr(WideStart,
                getMulExpr(WideMaxBECount,
                           getZeroExtendExpr(Step, WideTy)));
   if (SAdd == OperandExtendedAdd) {
     // If AR wraps around then
     //
     //    abs(Step) * MaxBECount > unsigned-max(AR->getType())
     // => SAdd != OperandExtendedAdd
     //
     // Thus (AR is not NW => SAdd != OperandExtendedAdd) <=>
     // (SAdd == OperandExtendedAdd => AR is NW)

     const_cast<SCEVAddRecExpr *>(AR)->setNoWrapFlags(SCEV::FlagNW);

     // Return the expression with the addrec on the outside.
     return getAddRecExpr(getSignExtendAddRecStart(AR, Ty, this),
                          getZeroExtendExpr(Step, Ty),
                          L, AR->getNoWrapFlags());
   }

Differential Revision: http://reviews.llvm.org/D7640

llvm-svn: 229594