[LA64_DYNAREC] Close the fastnan path for MINSS/MAXSS/VMINSS/VMAXSS to correct NaN semantics.

[yzewei]

Problem:
LoongArch's FMIN_S/FMAX_S rules for determining NaN are inconsistent with x86's MINSS/MAXSS.

This causes NaN handling errors when dynarec_fastnan=1 (error scenario observed: numpy max/argmax returns 1.0 instead of nan).

Test Code:

#gcc -O0 -mavx -o test test.c 
#include <immintrin.h>
#include <stdio.h>
#include <math.h>

int main() {
    float a = 1.0f;
    float b = NAN;

    __m128 va = _mm_set_ss(a);   // dest
    __m128 vb = _mm_set_ss(b);   // src
    __m128 vc = _mm_max_ss(va, vb); 

    float out;
    _mm_store_ss(&out, vc);
    printf("vmaxss out=%f\n", out);
    return 0;
}

If we need to check whether d0/d1 is NaN in order to preserve fastnan(FMAX_S), it might actually be slower.

[ptitSeb]

This is in par with ARM64 backend