[X86] Optimize v2i32/v2f32 scatters.

If the index is v2i64 we can use the scatter instruction that has v4i32/v4f32 data register, v2i64 index, and v2i1 mask. Similar was already done for gather.

Implement custom widening for v2i32 data to remove the code that reverses type legalization during lowering.

llvm-svn: 322254