[X86] Support v2i32/v4i16/v8i8 load/store using f64 on 32-bit targets under...

[X86] Support v2i32/v4i16/v8i8 load/store using f64 on 32-bit targets under -x86-experimental-vector-widening-legalization.

On 64-bit targets the type legalizer will use i64 to legalize these. But when i64 isn't legal, the type legalizer won't try an FP type. So do it manually instead.

There are a few regressions in here due to some v2i32 operations like mul and div now being reassembled into a full vector just to store instead of storing the pieces. But this was already occuring in 64-bit mode so its not a new issue.

llvm-svn: 346908