Fixed a bug in type legalizer for masked load/store intrinsics.

Fixed a bug in type legalizer for masked load/store intrinsics.
The problem occurs when after vectorization we have type
<2 x i32>. This type is promoted to <2 x i64> and then requires
additional efforts for expanding loads and truncating stores.
I added EXPAND / TRUNCATE attributes to the masked load/store
SDNodes. The code now contains additional shuffles.
I've prepared changes in the cost estimation for masked memory
operations, it will be submitted separately.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226808 91177308-0d34-0410-b5e6-96231b3b80d8