[SelectionDAGBuilder] Don't add base offset in LowerFormalArguments(). (#170732)
LowerCallTo() and LowerArguments() are both providing the PartOffset field for
each split argument part. As these two methods are intended to work together,
they should both provide the same offsets. However, LowerArguments() has been
providing the offset from the beginning of the struct while LowerCallTo() sets it
relative to the first split part.
This patch removes the PartBase variable in LowerArguments() so that the behavior
matches LowerCallTo(): offsets to split parts of an argument are relative to the first
part of the argument.