Add Block::to_vector

[abadams]

Blocks are guaranteed to be right-leaning, but several places were unpacking a Block tree into a vector of non-Block Stmts by hand, some of them recursively. This adds a single helper:

/** The inverse of the vector form of make. Unpacks a Stmt into the
 * sequence of non-Block Stmts it runs. An undefined Stmt unpacks to an
 * empty vector, and a non-Block Stmt unpacks to a vector of size one. */
static std::vector<Stmt> to_vector(const Stmt &s);

It's a static taking a Stmt rather than a member on Block, because every call site starts from a Stmt that may or may not be a Block, and that makes it a true inverse of Block::make(const std::vector<Stmt> &).

Call sites converted:

• LoopCarry had its own recursive block_to_vector pair; deleted.
• Prefetch had a recursive traverse_block template; deleted. HoistPrefetches::visit(Block) now collects into two vectors and concatenates.
• AsyncProducers, both in make_producer_consumer and in ExpandAcquireNodes::visit(Block).
• RemoveUndef::visit(Block). It no longer shares an unchanged suffix when something did change, but still returns op untouched when nothing changed, and still returns an undefined Stmt when everything was removed.
• StmtToHTML::print_block_stmt.

Left alone are the places that walk a block chain conditionally or partially rather than unpacking it: Simplify_Stmts, CodeGen_LLVM's leading-assert collection, LICM, Profiling, InjectHostDevBufferCopies, and Serialization.

test_correctness passes locally apart from the known LLVM 23 sve2 failure in simd_op_check.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.11%. Comparing base (c049b16) to head (dd9ecf9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9384      +/-   ##
==========================================
+ Coverage   70.03%   70.11%   +0.07%     
==========================================
  Files         261      261              
  Lines       79223    79190      -33     
  Branches    19312    19305       -7     
==========================================
+ Hits        55487    55524      +37     
+ Misses      17923    17893      -30     
+ Partials     5813     5773      -40     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[mcourteaux]

Not through a Block::make indeed. But someone could just set first and rest manually. An assert wouldn't hurt.