…g SNaNs.

Instead of ensuring outputs are arithmetic NaNs on every function, we tag them as pending such a check, so that a sequence of computation can have a single canonicalization step at the end.

There's an extra wriggle for SIMD. The Wasm type system only indicates them as V128, so it's possible that we might do computations as F32x4Add, I8x16Add, F64x2Add in a row with no other computations in between. Thus, most SIMD functions apply pending canonicalizations to their inputs, even integer SIMD operations.

…-nan-but-fast