Time for some belated spring cleaning!
This patch removes the offset parameter from @llvm.dbg.value a.k.a. DbgValueIntrinsic.
There is no situation where this rarely-used argument cannot be substituted with a DIExpression and removing it allows us to simplify the DWARF backend and bits of SelectionDAG.
Note that this patch does not yet remove any of the newly dead code. I'm planning to follow this up with a couple of smaller cleanup commits.