simd128: fix portable fallback for wasm_i8x16_swizzle
simd128: fix portable fallback for wasm_i8x16_swizzle

I broke it in the previous commit; it didn't properly account for the
possibility of the index being negative.

This also adds another set of braces in the NEON implementation, as the
old version didn't work on GCC on armv8.

nemequ/simde,nemequ/simde,nemequ/simde