[InstCombine] Canonicalize sadd.with.overflow with clamp to sadd.sat

This adds to D69245, adding extra signed patterns for folding from a
sadd_with_overflow to a sadd_sat. These are more complex than the
unsigned patterns, as the overflow can occur in either direction.

For the add case, the positive overflow can only occur if both of the
values are positive (same for both the values being negative). So there
is an extra select on whether to use the positive or negative overflow
limit.

Differential Revision: https://reviews.llvm.org/D69252