sql: reduce allocation in checkExprForDistSQL

[yuzefovich]

Here is a snippet of alloc_objects profile from 23.1.17:

ROUTINE ======================== github.com/cockroachdb/cockroach/pkg/sql.checkExpr in github.com/cockroachdb/cockroach/pkg/sql/distsql_physical_planner.go
1793616614 1793616614 (flat, cum)  1.24% of Total
         .          .    389:// supported by distSQL, like distSQL-blocklisted functions.
         .          .    390:func checkExpr(expr tree.Expr) error {
         .          .    391:	if expr == nil {
         .          .    392:		return nil
         .          .    393:	}
1793616614 1793616614    394:	v := distSQLExprCheckVisitor{}
         .          .    395:	tree.WalkExprConst(&v, expr)
         .          .    396:	return v.err
         .          .    397:}
         .          .    398:
         .          .    399:type distRecommendation int

I think we can reduce / eliminate allocations in checkExpr.

Jira issue: CRDB-37177