[Fix][Codegen] Preserve NaNs in floating-point min and max

[tlopex]

This PR fixes NaN handling in floating-point min and max code generation, partly fixed #19579

Previously, the ordered comparison caused a NaN in the left operand to be discarded, while a NaN in the right operand was preserved.

For floating-point operands, this PR implements the following semantics:

max: (a > b || isnan(a)) ? a : b
min: (a < b || isnan(a)) ? a : b

This preserves a NaN from either operand while retaining the existing behavior of selecting the second operand when the operands compare equal.

For the C host backend, a != a is used as the NaN check:

(a > b || a != a) ? a : b
(a < b || a != a) ? a : b

This compact form also produces better optimized code than the equivalent nested conditional expression.

For LLVM, the lowering combines an ordered comparison with an unordered self-comparison and emits a single select. This avoids relying on llvm.maximum/llvm.minimum, whose signed-zero semantics differ from the existing TVM behavior and whose legalization may vary across LLVM targets.

[gemini-code-assist]

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