winch: respect the enable_nan_canonicalization setting

[r-near]

The enable_nan_canonicalization flag already flows through to Winch via the shared Flags, but Winch was ignoring it. This adds a canonicalize_nan method to the Masm trait that, when the flag is set, emits a compare-with-self + conditional branch to replace NaN results with the canonical quiet NaN after each float arithmetic op.

Covered operations: add, sub, mul, div, min, max, sqrt, ceil, floor, trunc, nearest, demote, and promote. Implemented for x64 and aarch64. Includes a scalar wast test (counterpart to simd/canonicalize-nan.wast).

[Copilot]

Pull request overview

This PR ensures Winch respects the existing enable_nan_canonicalization shared setting by adding a canonicalize_nan hook to the MacroAssembler trait and invoking it after scalar floating-point operations so NaN results are normalized to the canonical quiet-NaN bit pattern.

Changes:

• Add MacroAssembler::canonicalize_nan and implement it for x64 and aarch64.
• Invoke NaN canonicalization after scalar float arithmetic/conversion ops (and after rounding paths in the visitor).
• Add a scalar WAST test to validate canonical NaN bit patterns.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
winch/codegen/src/visitor.rs	Inserts canonicalize_nan after scalar float ops and adds helper for rounding results.
winch/codegen/src/masm.rs	Extends the MacroAssembler trait with canonicalize_nan.
winch/codegen/src/isa/x64/masm.rs	Implements NaN detection + replacement with canonical NaN on x64.
winch/codegen/src/isa/aarch64/masm.rs	Implements NaN detection + replacement with canonical NaN on aarch64; stores shared flags in the masm.
tests/misc_testsuite/canonicalize-nan-scalar.wast	Adds scalar coverage for canonical NaN bit patterns and propagation.

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[r-near]

@cfallin Thanks for reviewing #12940! Would you mind taking a look at this one as well?

[cfallin]

@r-near we usually either stick with the assignment bot's suggestions (the reason I reviewed your last PR) or, in cases such as this one, tag a sub-area reviewer who knows part of the code best -- in this case @saulecabrera might be best to review Winch-specific work?

[saulecabrera]

I'll review.