Make PlaceHolderInfo lookup O(1).

Make PlaceHolderInfo lookup O(1).

Up to now we've just searched the placeholder_list when we want to
find the PlaceHolderInfo with a given ID.  While there's no evidence
of that being a problem in the field, an upcoming patch will add
find_placeholder_info() calls in build_joinrel_tlist(), which seems
likely to make it more of an issue: a joinrel emitting lots of
PlaceHolderVars would incur O(N^2) cost, and we might be building
a lot of joinrels in complex queries.  Hence, add an array that
can be indexed directly by phid to make the lookups constant-time.

Discussion: https://postgr.es/m/1405792.1660677844@sss.pgh.pa.us