minor MERGE cleanups

There's a complain from Coverity about outer_plan being referenced while
possibly NULL, which can be silenced by using an existing local
variable.  0001 does that.

0002 and 0003 are unrelated: in the former, we avoid creating a separate
local variable planSlot when we can just refer to the eponymous member
of ModifyTableContext.  In the latter, we reduce the scope where
'lockmode' is defined by moving it from ModifyTableContext to
UpdateContext, which means we can save initializing it in a few spots;
this makes the code more natural.

I expect these fixups in new code should be uncontroversial.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/