* [REFACTOR][IR] Introduce SeqStmt to replace Block
ir::Block was used to represent a sequence of Stmts in the original low-level IR.
The nested ir::Block structure is not really friendly for recursive visits,
especially when the statements are unrolled.
This PR introduce a SeqStmt that directly stores a sequence of statements in an Array container.
The new SeqStmt will be used as a replacement of the original Block structure.
* [REFACTOR] Migrate use of Block to SeqStmt.
* [REFACTOR] Remove Block
* Add more comments per yizhi's comment
