ART: Change merges with Undefined to Undefined
The result of a merge with an Undefined type should be Undefined.
Conflicts are allowed to be copied around, but Undefined registers
should not be touched at all, except to be written into.
Add a success test case (the register isn't used) and a fail test
case (the register is tried to be copied).
(cherry picked from commit 97a1ff353f254b6e46c7501fe3f0e3254c2517b4)
5 files changed