[InstCombine] Use known overflow information for saturating add/sub

If ValueTracking can determine that the add/sub can newer overflow,
replace it with the corresponding nuw/nsw add/sub.

Additionally, for the unsigned case, if ValueTracking determines
that the add/sub always overflows, replace the result with the
saturation value.

This change is part of https://reviews.llvm.org/D54534.

llvm-svn: 347770