Replace EEOP_DONE with special steps for return/no return

Replace EEOP_DONE with special steps for return/no return

Knowing when the side-effects of an expression is the intended result
of the execution, rather than the returnvalue, is important for being
able generate more efficient JITed code. This replaces EEOP_DONE with
two new steps: EEOP_DONE_RETURN and EEOP_DONE_NO_RETURN.  Expressions
which return a value should use the former step; expressions used for
their side-effects which don't return value should use the latter.

Author: Andres Freund <[email protected]>
Co-authored-by: Daniel Gustafsson <[email protected]>
Reviewed-by: Andreas Karlsson <[email protected]>
Discussion: https://postgr.es/m/415721CE-7D2E-4B74-B5D9-1950083BA03E@yesql.se
Discussion: https://postgr.es/m/20191023163849[email protected]