[DAGCombiner] use root SDLoc for all nodes created by logic fold

If this is not a valid way to assign an SDLoc, then we get this
wrong all over SDAG.

I don't know enough about the SDAG to explain this. IIUC, theoretically,
debug info is not supposed to affect codegen. But here it has clearly
affected 3 different targets, and the x86 change is an actual improvement.

llvm-svn: 348552