[SCEV] Keep common NUW flags when inlining Add operands.

Currently, NoWrapFlags are dropped if we inline operands of SCEVAddExpr
operands. As a consequence, we always drop flags when building
expressions like `getAddExpr(A, getAddExpr(B, C, NUW), NUW)`.

We should be able to retain NUW flags common among all inlined
SCEVAddExpr and the original flags.

Reviewed By: nikic, mkazantsev

Differential Revision: https://reviews.llvm.org/D103877