[ValueTracking] Use ConstantRange overflow check for signed add; NFC

This is the same change as rL356290, but for signed add. It replaces
the existing ripple logic with the overflow logic in ConstantRange.

This is NFC in that it should return NeverOverflow in exactly the
same cases as the previous implementation. However, it does make
computeOverflowForSignedAdd() more powerful by now also determining
AlwaysOverflows conditions. As none of its consumers handle this yet,
this has no impact on optimization. Making use of AlwaysOverflows
in with.overflow folding will be handled as a followup.

Differential Revision: https://reviews.llvm.org/D59450

llvm-svn: 356345