[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