Refactor parser: Avoid some assertion for StmtNode

[shenli]

Make code cleaner. No performance improvement.

Before:

➜ parser git:(master) ✗ go test -run=XXX -bench=. -benchtime=10s
BenchmarkSysbenchSelect-4 3000000 4589 ns/op 1888 B/op 27 allocs/op
BenchmarkParse-4 1000000 14310 ns/op 5224 B/op 89 allocs/op
PASS
ok github.com/pingcap/tidb/parser 32.910s
➜ parser git:(master) ✗ go test -run=XXX -bench=. -benchtime=10s
BenchmarkSysbenchSelect-4 3000000 4770 ns/op 1888 B/op 27 allocs/op
BenchmarkParse-4 1000000 14001 ns/op 5224 B/op 89 allocs/op
PASS
ok github.com/pingcap/tidb/parser 33.489s
➜ parser git:(master) ✗ go test -run=XXX -bench=. -benchtime=10s
BenchmarkSysbenchSelect-4 3000000 4679 ns/op 1888 B/op 27 allocs/op
BenchmarkParse-4 ^[[A 1000000 14028 ns/op 5224 B/op 89 allocs/op
PASS
ok github.com/pingcap/tidb/parser 32.791s
➜ parser git:(master) ✗ go test -run=XXX -bench=. -benchtime=10s
BenchmarkSysbenchSelect-4 3000000 4628 ns/op 1888 B/op 27 allocs/op
BenchmarkParse-4 1000000 14434 ns/op 5224 B/op 89 allocs/op
PASS
ok github.com/pingcap/tidb/parser 33.395s

After:

➜ parser git:(shenli/parser) ✗ go test -run=XXX -bench=. -benchtime=10s
BenchmarkSysbenchSelect-4 3000000 4586 ns/op 1888 B/op 27 allocs/op
BenchmarkParse-4 1000000 14229 ns/op 5224 B/op 89 allocs/op
PASS
ok github.com/pingcap/tidb/parser 32.819s
➜ parser git:(shenli/parser) ✗ go test -run=XXX -bench=. -benchtime=10s
BenchmarkSysbenchSelect-4 3000000 4567 ns/op 1888 B/op 27 allocs/op
BenchmarkParse-4 1000000 14253 ns/op 5224 B/op 89 allocs/op
PASS
ok github.com/pingcap/tidb/parser 32.768s
➜ parser git:(shenli/parser) ✗ go test -run=XXX -bench=. -benchtime=10s
BenchmarkSysbenchSelect-4 3000000 4622 ns/op 1888 B/op 27 allocs/op
BenchmarkParse-4 1000000 14979 ns/op 5224 B/op 89 allocs/op
PASS
ok github.com/pingcap/tidb/parser 33.719s
➜ parser git:(shenli/parser) ✗ go test -run=XXX -bench=. -benchtime=10s
BenchmarkSysbenchSelect-4 3000000 4701 ns/op 1888 B/op 27 allocs/op
BenchmarkParse-4 1000000 14687 ns/op 5224 B/op 89 allocs/op
PASS
ok github.com/pingcap/tidb/parser 33.645s
➜ parser git:(shenli/parser) ✗ go test -run=XXX -bench=. -benchtime=10s
BenchmarkSysbenchSelect-4 3000000 4561 ns/op 1888 B/op 27 allocs/op
BenchmarkParse-4 1000000 14233 ns/op 5224 B/op 89 allocs/op
PASS
ok github.com/pingcap/tidb/parser 32.730s

[shenli]

/run-all-tests

[zz-jason]

LGTM

[XuHuaiyu]

align these 5 lines.

[XuHuaiyu]

why we add type assertion here?

[shenli]

$1.(*ast.SubqueryExpr).Query is a ResultSet, not StmtNode.

[shenli]

@XuHuaiyu PTAL

[XuHuaiyu]

circleci failed @shenli

[shenli]

@XuHuaiyu I can not open CircleCI now. I merge master and rerun it.

[XuHuaiyu]

LGTM