[ValueTracking] More accurate unsigned add overflow detection

[ValueTracking] More accurate unsigned add overflow detection

Part of D58593.

Compute precise overflow conditions based on all known bits, rather
than just the sign bits. Unsigned a + b overflows iff a > ~b, and we
can determine whether this always/never happens based on the minimal
and maximal values achievable for a and ~b subject to the known bits
constraint.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@355072 91177308-0d34-0410-b5e6-96231b3b80d8