[Codegen] Split StableHLO/JAX select-style argmax reductions (#24445)

Problem:
StableHLO/JAX lowers argmax for lm_head vocab reductions as a pair of
selects with explicit NaN handling and lower-index tie breaking. The
existing split-reduction matcher only recognized the canonical
maximumf/cmpf/select form, so large vocab reductions stayed as one wide
reduction.

Fix:
Teach the argmax matcher to distinguish canonical and StableHLO/JAX
select-style argmax reductions, and preserve the corresponding combiner
when splitting. Keep the select-style matcher strict by requiring the
`gt || isnan` value condition, the `eq && lower_index` tie condition,
and the expected value/index select operands.

Example:
A Qwen `lm_head` argmax over vocab size 248320 can now split into a
two-stage reduction over 1940x128 tiles while preserving NaN and
lower-index tie semantics. Negative tests ensure similar-looking select
reductions with different NaN/tie/index semantics do not split.

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Signed-off-by: Yuwei Sun <yuweis2@illinois.edu>
Signed-off-by: Hao Ren <rhao8608@gmail.com>
Co-authored-by: Hao Ren <rhao8608@gmail.com>