[Bug tree-optimization/107096] Fully masking vectorization with AVX512 ICEs gcc.dg/vect/vect-over-widen-*.c

["crazylht at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107096

--- Comment #11 from Hongtao.liu <crazylht at gmail dot com> ---

> 
> There's no other way to do N bit to two N/2 bit hi/lo (un)packing
> (there's a 2x N/2 bit -> N bit operation, for whatever reason).
> There's also no way to transform the d rgroup mask into the
> f rgroup mask for the first example aka duplicate bits in place,
> { b0, b1, b2, ... bN } -> { b0, b0, b1, b1, b2, b2, ... bN, bN },
> nor the reverse.
> 

Can we just do VIEW_CONVERT_EXPR for vectype instead of mask_type.
.i.e
we can do VCE to tranform V8SI to V16HI, then use mask_load for V16HI with same
mask {b0, b0, b1, b1, b2, b2, .}, then VCE it to back to V8SI, it should be ok
as long as duplicated bits in place.(or VCE V16HI to V8SI then use mask {b0,
b1, b2, ..., bN}, and VCE V8SI back to V16HI after masked load/move).