[DAGCombiner] Set the right SDLoc on extended SETCC uses (7/N)

ExtendSetCCUses updates SETCC nodes which use a load (OriginalLoad) to
reflect a simplification to the load (ExtLoad).

Based on my reading, ExtendSetCCUses may create new nodes to extend a
constant attached to a SETCC. It also creates fresh SETCC nodes which
refer to any updated operands.

ISTM that the location applied to the new constant and SETCC nodes
should be the same as the location of the ExtLoad.

This was suggested by Adrian in https://reviews.llvm.org/D45995.

Part of: llvm.org/PR37262

Differential Revision: https://reviews.llvm.org/D46216

llvm-svn: 332119