[SelectionDAG] Teach BaseIndexOffset::match to unwrap the base after looking through an add/or

We might find a target specific node that needs to be unwrapped after we look through an add/or. Otherwise we get inconsistent results if one pointer is just X86WrapperRIP and the other is (add X86WrapperRIP, C)

Differential Revision: https://reviews.llvm.org/D54818

llvm-svn: 347591