Refactor the checks for parameterized partial paths

Refactor the checks for parameterized partial paths

Parameterized partial paths are not supported, and we have several
checks in try_partial_xxx_path functions to enforce this.  For a
partial nestloop join path, we need to ensure that if the inner path
is parameterized, the parameterization is fully satisfied by the
proposed outer path.  For a partial merge/hashjoin join path, we need
to ensure that the inner path is not parameterized.  In all cases, we
need to ensure that the outer path is not parameterized.

However, the comment in try_partial_hashjoin_path does not describe
this correctly.  This patch fixes that.

In addtion, this patch simplifies the checks peformed in
try_partial_hashjoin_path and try_partial_mergejoin_path with the help
of macro PATH_REQ_OUTER, and also adds asserts that the outer path is
not parameterized in try_partial_xxx_path functions.

Author: Richard Guo
Discussion: https://postgr.es/m/CAMbWs48mKJ6g_GnYNa7dnw04MHaMK-jnAEBrMVhTp2uUg3Ut4A@mail.gmail.com