Replace a few uses of SelectionDAG::getTargetNode with

SelectionDAG::SelectNodeTo in the instruction selector. This
updates existing nodes in place instead of creating new ones.

Go back to selecting ISD::DBG_LABEL nodes into
TargetInstrInfo::DBG_LABEL nodes instead of leaving them
unselected, now that SelectNodeTo allows us to update them
in place.

llvm-svn: 53057