opt: prune columns that are guaranteed not to be needed for partial index mutations

[mgartner]

In the example below, a does not need to be fetched because the secondary partial index is guaranteed not to change during this mutation—none of its indexed or referenced columns are changing.

Also, the partial index put and del columns can be pruned.

root@127.0.0.1:54758/defaultdb> create table t (k int primary key, a int, b int, c int, index (a) where b > 1, family (a), family (b), family (c));
CREATE TABLE

Time: 3.244ms

root@127.0.0.1:54758/defaultdb> explain (opt, verbose) update t set c = c + 1 where true;
                                       text
----------------------------------------------------------------------------------
  update t
   ├── columns: <none>
   ├── fetch columns: k:5 a:6 c:8
   ├── update-mapping:
   │    └── c_new:10 => c:4
   ├── partial index put columns: partial_index_del1:9
   ├── partial index del columns: partial_index_del1:9
   ├── cardinality: [0 - 0]
   ├── volatile, mutations
   ├── stats: [rows=0]
   ├── cost: 1190.04
   └── project
        ├── columns: c_new:10 partial_index_del1:9 k:5 a:6 c:8
        ├── immutable
        ├── stats: [rows=1000]
        ├── cost: 1190.03
        ├── key: (5)
        ├── fd: (5)-->(6,8-10), (8)-->(10)
        ├── prune: (5,6,8-10)
        ├── scan t
        │    ├── columns: k:5 a:6 b:7 c:8
        │    ├── partial index predicates
        │    │    └── t_a_idx: filters
        │    │         └── b:7 > 1 [outer=(7), constraints=(/7: [/2 - ]; tight)]
        │    ├── stats: [rows=1000]
        │    ├── cost: 1160.02
        │    ├── key: (5)
        │    ├── fd: (5)-->(6-8)
        │    └── prune: (5-8)
        └── projections
             ├── c:8 + 1 [as=c_new:10, outer=(8), immutable]
             └── b:7 > 1 [as=partial_index_del1:9, outer=(7)]