[DAGCombiner] Set the right SDLoc on a newly-created sextload (6/N)

This teaches tryToFoldExtOfLoad to set the right location on a
newly-created extload. With that in place, the logic for performing a
certain ([s|z]ext (load ...)) combine becomes identical for sexts and
zexts, and we can get rid of one copy of the logic.

The test case churn is due to dependencies on IROrders inherited from
the wrong SDLoc.

Part of: llvm.org/PR37262

Differential Revision: https://reviews.llvm.org/D46158

llvm-svn: 332118