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

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

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

Follow-ups will include extending this to more
OverflowingBinaryOperators.

Depends on D142387.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D142390