[mlir][sparse] refactor loop sequence codegen

This refactoring adds a few "event" functions (start/end loop-seq/loop) for
readability of the core function of codegen. This also prepares sparse tensor
output codegen, where these "event" functions will provide convenient
placeholders to start or stop insertion bookkeeping.

This revision also includes a few various minor changes that kept on
pending in my local workspace.

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D112506