The fallback (which RISC-V uses) did its integer arithmetic in plain int,
where signed overflow and left shifts of negative values are undefined,
and converted out of range floats with a cast, which is undefined too:
RISC-V gives INT_MAX for NaN where SSE gives INT_MIN and NEON 0. With GCC
on RISC-V that broke GEUVSpanCore in the GEMath unit test. Now the
arithmetic wraps through uint32_t, like the SIMD instructions, and an out
of range or NaN conversion gives INT_MIN, like SSE.
Also skip GERowSum's zero terms before computing their shift, as their
exponent can be INT_MIN. Found with -fsanitize=undefined on the fallback.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
