[DAGCombiner] Fix SDLoc in a (sext (sextload x)) combine (3/N)

Prior to this patch, for the given test case, we would apply the
location associated with the sdiv instruction to instructions which
perform the load.

Part of: llvm.org/PR37262.

Differential Revision: https://reviews.llvm.org/D46222

llvm-svn: 331302