enable point get when plan cache is turned on

[eurekaka]

Feature Request

Is your feature request related to a problem? Please describe:

Currently, when plan cache is enabled, point get is disabled for all cases:

func TryFastPlan(ctx sessionctx.Context, node ast.Node) Plan {
    if PreparedPlanCacheEnabled() {
        // Do not support plan cache.
        return nil
    }
    switch x := node.(type) {
    case *ast.SelectStmt:
        fp := tryPointGetPlan(ctx, x)
        if fp != nil {
            if checkFastPlanPrivilege(ctx, fp, mysql.SelectPriv) != nil {
                return nil
            }
            return fp
        }
    case *ast.UpdateStmt:
        return tryUpdatePointPlan(ctx, x)
    case *ast.DeleteStmt:
        return tryDeletePointPlan(ctx, x)
    }
    return nil
}

However, according to Cacheable, we would not cache plan for DELETE/UPDATE at all, so we can enable point get for UPDATE/DELETE at least.

Describe the feature you'd like:

Try point get for DELETE/UPDATE when plan cache is enabled for better performance.

Describe alternatives you've considered:

Remove the PreparedPlanCacheEnabled() check at all for TryFastPlan. Will check whether it is safe later.

Teachability, Documentation, Adoption, Migration Strategy:

N/A

[eurekaka]

After enhancing rebuildRange, point get for select is cacheable now, so we can just remove PreparedPlanCacheEnabled() check for TryFastPlan in total.