fix(simplifier): VectorReduce(Broadcast(Vector)) used wrong semantics of Broadcast(Vector).

[mcourteaux]

• fix(simplifier): VectorReduce(Broadcast(Vector)) used wrong semantics of Broadcast(Vector). To review this; first read the modified test to understand easier what's changed. The bug was was introduced by @dsharlet in e164866 from Add simplification rules for vectorreduce of broadcasts #5372. @abadams approved that PR. However, the documentation today states that a broadcast of a vector is a concatenation of N times said vector:
  

  Halide/src/IR.h

  Lines 267 to 271 in bce61d9

  	/** A vector with 'lanes' elements, in which every element is 'value'. This is a
  	* special case of the ramp node above, in which the stride is zero. If the
  	* value is a vector, this is the concatenation of 'lanes' repeated copies of
  	* that vector. */
  	struct Broadcast : public ExprNode<Broadcast> {

  
  The changes in Add simplification rules for vectorreduce of broadcasts #5372 interpret a broadcast of a vector as an interleave of N times the vector (meaning the lanes are repeated).
• fix(simplifier): Cherry-picked fix by @abadams in Fix Vulkan interleave two vectors bug + Vector Legalization lowering pass #8629.

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

[abadams]

I think this (and the max one) could be more general:

rewrite(h_min(ramp(x, y, arg_lanes), 1), x + min(y * (arg_lanes - 1), 0)) ||
rewrite(h_min(ramp(x, y, arg_lanes), lanes), ramp(x + min(y * (factor - 1), 0), y * factor, lanes)) ||

Not sure if the more general case would ever come up though. Maybe it's not worth it.

[mcourteaux]

With some assistance of Claude to break it down, I am convinced too that these more general rules are good. I'll add them, along with some tests.

[mcourteaux]

Added them! Code formatting looks a lot better. 🥳