HPCC-10361 Prevent assignements that can SKIP from being lost

Assignments that contained skips could be lost from a transform if it was merged
with a subsequent transform, and that transform didn't use the field the assignment
targetted.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5178 from ghalliday/issue10361

HPCC-10361 Prevent assignments that can SKIP from being lost

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>