Consider materializing the cheapest inner path in parallel nestloop

Consider materializing the cheapest inner path in parallel nestloop

When generating non-parallel nestloop paths for each available outer
path, we always consider materializing the cheapest inner path if
feasible.  Similarly, in this patch, we also consider materializing
the cheapest inner path when building partial nestloop paths.  This
approach potentially reduces the need to rescan the inner side of a
partial nestloop path for each outer tuple.

Author: Tender Wang
Reviewed-by: Richard Guo, Robert Haas, David Rowley, Alena Rybakina
Reviewed-by: Tomasz Rybak, Paul Jungwirth, Yuki Fujii
Discussion: https://postgr.es/m/CAHewXNkPmtEXNfVQMou_7NqQmFABca9f4etjBtdbbm0ZKDmWvw@mail.gmail.com