[ValueTracking] Use ConstantRange overflow checks for unsigned add/sub; NFC

Use the methods introduced in rL356276 to implement the
computeOverflowForUnsigned(Add|Sub) functions in ValueTracking, by
converting the KnownBits into a ConstantRange.

This is NFC: The existing KnownBits based implementation uses the same
logic as the the ConstantRange based one. This is not the case for the
signed equivalents, so I'm only changing unsigned here.

This is in preparation for D59386, which will also intersect the
computeConstantRange() result into the range determined from KnownBits.

llvm-svn: 356290