[DAGCombiner] Support {a|s}ext, {a|z|s}ext load nodes in load combine

[DAGCombiner] Support {a|s}ext, {a|z|s}ext load nodes in load combine

Resubmit r295336 after the bug with non-zero offset patterns on BE targets is fixed (r296336).

Support {a|s}ext, {a|z|s}ext load nodes as a part of load combine patters.

Reviewed By: filcab

Differential Revision: https://reviews.llvm.org/D29591

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@296651 91177308-0d34-0410-b5e6-96231b3b80d8