[APInt] Add methods for saturated add and sub

This adds the sadd_sat, uadd_sat, ssub_sat, usub_sat methods for performing saturating additions and subtractions to APInt.

Split out from D54237.

Patch by: nikic (Nikita Popov)

Differential Revision: https://reviews.llvm.org/D54332

llvm-svn: 347324