Why does `PBToExpr` build new field type but not use the existing one in pb?

## General Question

The function `(github.com/pingcap/tidb/expression).PBToExpr` receives an argument: `expr *tipb.Expr`. Some of the branches use the field type in `expr.FieldType`, but some of them (e.g. `ExprType_Int64`...) doesn't use, but build a new one through `types.NewFieldType(mysql.TypeLongLong)`.

Is it correct to always use the field type converted from `PbTypeToFieldType`? As the current implementation will loss all flag information.

In #37404, I'll use `PbTypeToFieldType` for `Int64`. If it's right to always use `PbTypeToFieldType` to get the type (rather than build a new one), I'll submit another PR to migrate all of them.