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

Resubmit -r295314 with PowerPC and AMDGPU tests updated.

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

llvm-svn: 295336