[alexcgatea]

Summary

Adds the Rewriter operations for code motion: moving statements, splicing a block's statements, and splicing a whole CFG. Walks now detect when the statement being visited is moved.

New operations

Operation	What it does
replace_uses_if, replace_all_uses_except_stmt(s)	replace_all_uses restricted to selected use sites
move_before / move_after	Move a statement next to an anchor in any block body
splice_block_before / splice_block_after	Move a block's statements next to an anchor, redirecting its arguments
splice_cfg_after	Move all blocks of a CFG after an anchor block in another CFG

Each records a new event (MovedStatement, SplicedBlock, SplicedCfg). Moving a statement into its own bodies, or splicing a block or CFG into itself, is rejected (MovedIntoItself, SplicedIntoItself, CfgSplicedIntoItself).

API changes

• drain_events is replaced by into_events(self), so the event log only grows while a Rewriter lives, and positions in it stay valid.

Walk safety

Moving or splicing the statement being visited breaks pre-order, post-order and BFS walks, which find the next statement through the visited one. walk_mut now checks the Rewriter's event log after each visit (MovedStatement, SplicedBlock, SplicedCfg, compared against a VisitedPlace recorded before the visit) and reports WalkError::VisitedMoved. Reverse BFS orders, which build their order up front, still allow it. A loop driving a cursor by hand records VisitedPlace::of(stage, stmt) and the event-log length before each visit, and calls WalkOrder::check_not_moved_since itself.