Fixed a bug in splitting Scatter operation in the Type Legalizer.

After the split of the Scatter operation, the order of the new instructions is well defined - Lo goes before Hi. Otherwise the semantic of Scatter (from LSB to MSB) is broken.
I'm chaining 2 nodes to prevent reordering.

Differential Revision https://reviews.llvm.org/D37670

llvm-svn: 312894