[SCCP] Use range info to prove AddInst has NUW flag.

This patch updates SCCP to use the value ranges of AddInst operands to
try to prove the AddInst does not overflow in the unsigned sense and
adds the NUW flag. The reasoning is done with
makeGuaranteedNoWrapRegion (thanks @nikic for point it out!).

Follow-ups will include adding NSW and extension to more
OverflowingBinaryOperators.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D142387