ARM: ensure fixed-point conversions have sane types

We were generating intrinsics for NEON fixed-point conversions that didn't
exist (e.g. float -> i16). There are two cases to consider:
  + iN is smaller than float. In this case we can do the conversion but need an
    extend or truncate as well.
  + iN is larger than float. In this case using the NEON conversion would be
    incorrect so we don't perform any combining.