Finish `Statement` rework. (dotnet/coreclr#26876)

Finish `Statement` rework. (dotnet/coreclr#26876)

* Delete a leftover for STMT and update dump examples.

* Prepare `GetNextStmt` and `GetPrevStmt` to be used as main read accesors.

* Rename `Statement->gtNext` to `m_nextStmt` and make private.

* Rename `Statement->gtPrev` to `m_prev` and make private.

* Rename `gtStmtLastILoffs` to `m_lastILOffset` and make it private.

* Rename `gtStmtILoffsx` to `m_ILOffsetX` and make it private.

* Rename `compilerAdded` to `m_compilerAdded` and make it private.

* Rename `gtInlineContext` to `m_inlineContext` and make it private.

* Rename `gtStmtList` to `m_treeList` and make it private.

* Rename `gtStmtExpr` to `m_rootTree` and make it private.

* Update the class layout acording to our spec.

* Update the docs.

* Delete another forgotten line.

* Rename `m_rootTree` to `m_TreeRoot`.

Keep it consistent with `m_treeList` .

* Rename `m_TreeRoot` to `m_rootNode`.

* Add comments.

Commit migrated from https://github.com/dotnet/coreclr/commit/e0f4a6477f1656648f3469adf4a1d0bbc1c1e60d