[VectorDistribute] Refactor VectorLayoutAnalysis into 2-phase forward/backward design (#23611)

Restructure the layout analysis from an interleaved forward+backward
worklist into a clean two-phase design:

Phase 1 (forward): Multi-candidate propagation from ToLayoutOp anchors
through uses. No IR mutation.

Resolve: Pick first candidate per value. This is a placeholder cost
model for now which matches the old analysis. Eventually, we will
consider coalescing, compute ops, mma layout, etc.

Phase 2 (backward fixup): Walk operations in reverse program order via
recursive fixupRegion/fixupOp. For each op, derive operand layouts from
resolved result layouts. Assign missing layouts, clone cheap ops
(constants, create_mask, step), or insert to_layout conversions on
conflict.

This naturally handles conflicts better in a predictable manner. // The
forward analysis is the main driver of the analysis. The reason for this
is that for a program to be well-formed for vector distribution, there
must be some way for the final store/return to get a layout. Otherwise,
there is not enough information in the program to determine how
distribution should be done. The forward analysis ensures that the final
return/store gets a layout in a well-formed program. The rest of the
program can get their layouts from backward propagation, everything in
the program must eventually reach the store/return.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>