Pushing back on implicit converions for simd types

[DenisYaroshevskiy]

I had seen the other discussion but I think it misses that simd conversions can be very expensive.

Here is 2 chars to double https://godbolt.org/z/fGnMdYr7h

        pmovsxbq        xmm1, xmm0
        movd    eax, xmm1
        xorps   xmm0, xmm0
        cvtsi2sd        xmm0, eax
        pextrd  eax, xmm1, 2
        xorps   xmm1, xmm1
        cvtsi2sd        xmm1, eax
        unpcklpd        xmm0, xmm1                      # xmm0 = xmm0[0],xmm1[0]
        ret

4 chars to double will be much worse because needs to emulate double.