[DAGCombiner] add enum for store source value; NFC

This removes existing code duplication and allows us to
assert that we are handling the expected cases.

We have a list of outstanding bugs that could benefit by
handling truncated source values, so that's a possible
addition going forward.

GitOrigin-RevId: 39329d5724d94737fda0212f8e89ca240f14474a
1 file changed