[SelectionDAG] make getMemBasePlusOffset() accessible; NFCI

I reinvented this functionality in http://reviews.llvm.org/D16828 because it was
hidden away as a static function. The changes in x86 are not based on a complete
audit. I suspect there are other possible uses there, and there are almost certainly
more potential users in other targets.

llvm-svn: 260295