Remove aligned/unaligned load/store fragments defined in MipsInstrInfo.td and

Remove aligned/unaligned load/store fragments defined in MipsInstrInfo.td and
use load/store fragments defined in TargetSelectionDAG.td in place of them.
Unaligned loads/stores are either expanded or lowered to target-specific nodes,
so instruction selection should see only aligned load/store nodes.

No changes in functionality.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163960 91177308-0d34-0410-b5e6-96231b3b80d8