Replacing lfirst() with lfirst_node() appropriately in planner.c

Hi,

Happened to stumble across some instances of lfirst() which could use
lfirst_node() in planner.c. Here's patch which replaces calls to
lfirst() extracting node pointers by lfirst_node() in planner.c. I
have covered all the occurences of lfirst() except
1. those extract generic pointers like Path, Plan etc.
2. those extract any node as Aggref, Var and then check using IsA()
3. those extracting some pointers other than nodes.

"make check" runs without any failure.

Are we carrying out such replacements in master or this will be
considered for v11?

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Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company