[X86] Fix bug in legalize vector types - Split large loads

[X86] Fix bug in legalize vector types - Split large loads

When splitting a large load to smaller legally-typed loads, the last load should be padded to reach the size of the previous one so a CONCAT_VECTORS node could reunite them again.
The code currently pads the last load to reach the size of the first load (instead of the previous).

Differential Revision: https://reviews.llvm.org/D38495

Change-Id: Ib60b55ed26ce901fabf68108daf52683fbd5013f

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317206 91177308-0d34-0410-b5e6-96231b3b80d8