HPCC-10922 Add an option to improve cse for if conditions in child queries

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5504 from ghalliday/issue10922

HPCC-10922 Add an option to improve cse for if conditions in child queries

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>