[X86] Custom type legalize v2i32/v4i16/v8i8->i64 bitcasts in 64-bit mode...

[X86] Custom type legalize v2i32/v4i16/v8i8->i64 bitcasts in 64-bit mode similar to what's done when the destination is f64.

The generic legalizer will fall back to a stack spill that uses a truncating store. That store will get expanded into a shuffle and non-truncating store on pre-avx512 targets. Once that happens the stack store/load pair will be combined away leaving behind the shuffle and bitcasts. On avx512 targets the truncating store is legal so doesn't get folded away.

By custom legalizing it we can avoid this churn and maybe produce better code.

llvm-svn: 348085