EnzymeHLOOpt: DUSDUSSubsuming moves slices only when it replaces the DUS

[wsmoses]

DUSDUSSubsuming, on two stacked dynamic update slices, does two things:

1. It moves slices of the inner update whose values don't change onto the outer one, so the inner update dies once the outer one is replaced.
2. It replaces the outer update with a piecewise select.

It reported success if either happened. When the outer update can't be replaced (here: partial updates whose sources don't span the result), it still moved the slices. SliceOfDynamicUpdate then moves each slice back, since a slice of a DUS that's clear of its update is a slice of the DUS's operand. The two patterns traded the slices forever:

APPLIED DUSDUSSubsuming on stablehlo.dynamic_update_slice
APPLIED SliceOfDynamicUpdate on stablehlo.slice
APPLIED SliceOfDynamicUpdate on stablehlo.slice
APPLIED DUSDUSSubsuming on stablehlo.dynamic_update_slice
...

Fix: make the replacement first and move the slices only when it's made. rematerializeByPiecewiseSelect returns null before creating any op when it can't build the replacement, so trying it first leaves the IR untouched on failure.

Found via: MFEM bilininteg_vecdiffusion_pa.cpp. With #3444, one function's enzyme-hlo-opt never finished (> 10 min); with this fix it takes about a second. The cycle is in main's patterns: main hangs on the IR #3444 produces there, and on the 8-line test added here.

Test: new dusdus_slice_clear_of_update.mlir with full-line goldens. Main doesn't terminate on it; with this fix the IR is left unchanged. All 42 *dus* lit tests pass. Full local lit: 1506/1509. The other three fail on main as well: mem2reg_copied_field and mem2reg_split_aggregate (fixed by #3471), and //test:neuralgcm_test, which runs out of memory on our 32 GB GPUs.

🤖 Generated with Claude Code

https://claude.ai/code/session_016zErYp7upmqr4NHfhod9UD

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 29.98%. Comparing base (fb49aed) to head (54db176).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3470      +/-   ##
==========================================
+ Coverage   29.92%   29.98%   +0.06%     
==========================================
  Files         241      241              
  Lines       48628    48670      +42     
==========================================
+ Hits        14551    14596      +45     
+ Misses      34077    34074       -3     

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