Let standard casting / builtin_convertvector handle the conversions from BF16 to F32
My only query is how to best implement _mm_cvtpbh_ps - I went for the
v8bf16 -> v8f32 conversion followed by subvector extraction in the end,
but could just as easily extract a v4bf16 first - makes no difference to
final optimized codegen.
First part of #154911
fca41f4aa1