rfc + [autovect] [patch] vectorization of type conversions

[Dorit Naishlos]

>
> For vectorized NOP_EXPR that does a type-promotion I added the following
> new tree-codes:
> VEC_UNPACK_HI_EXPR, VEC_UNPACK_LO_EXPR
> and the following new optabs: vec_unpack_hi_optab, vec_unpack_lo_optab.
>
> For vectorized NOP_EXPR that does a type-demotion I added the following
new
> tree-codes:
> VEC_PACK_MOD_EXPR, VEC_PACK_SAT_EXPR
> and the following new optabs: vec_pack_mod_optab, vec_pack_sat_optab.
> (These tree-codes are not being generated yet by the vectorizer).
>

by the way, I would actually probably need separate optabs for the signed
and unsigned case for all of these.

dorit