[DAGCombiner] DAG combine does not know how to combine indexed loads with

[DAGCombiner] DAG combine does not know how to combine indexed loads with
sign/zero/any extensions. However a few places were not checking properly the
property of the load and were turning an indexed load into a regular extended
load. Therefore the indexed value was lost during the process and this was
triggering an assertion.

<rdar://problem/16389332>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205923 91177308-0d34-0410-b5e6-96231b3b80d8