[DAG] Allow AssertZExt to scalarize. (#122463)
With range and undef metadata on a call we can have vector AssertZExt
generated on a target with no vector operations. The AssertZExt needs to
scalarize to a normal `AssertZext tin, ValueType`. I have added
AssertSext too, although I do not have a test case.
Fixes #110374
(cherry picked from commit ab9a80a3ad78f611fd06cd6f7215bd828809310c)
