x86-64: correct AVX512F vcvtsi2s{d,s} handling

Just like for their AVX counterparts and CVTSI2S{D,S}, a memory source
here is ambiguous and hence
- in source files should be qualified with a suitable suffix or operand
  size specifier (not doing so is an error in Intel mode, and will gain
  a diagnostic in AT&T mode in the future),
- in disassembly should be properly suffixed (the Intel operand size
  specifiers were emitted correctly already).