[Bug target/107432] __builtin_convertvector generates inefficient code

[crazylht at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107432

--- Comment #6 from Hongtao.liu <crazylht at gmail dot com> ---

> Guess expand_vector_conversion can be optimized.

  if (INTEGRAL_TYPE_P (TREE_TYPE (ret_type))
      && SCALAR_FLOAT_TYPE_P (TREE_TYPE (arg_type)))
    code = FIX_TRUNC_EXPR;
  else if (INTEGRAL_TYPE_P (TREE_TYPE (arg_type))
           && SCALAR_FLOAT_TYPE_P (TREE_TYPE (ret_type)))
    code = FLOAT_EXPR;

It only supports floatmn2/fix_truncmn2 for float <-> integer.

But we can also supports extendmn2/zero_extendmn2/truncmn2 for float <-> float,
integer <-> integer.

Or are there any concerns and VEC_PACK_TRUNC_EXPR,
VEC_PACK_FIX_TRUNC_EXPR,VEC_PACK_FLOAT_EXPR are used on purpose?