[SelectionDAG] Make getUniformBase in SelectionDAGBuilder fail if any of the...

[SelectionDAG] Make getUniformBase in SelectionDAGBuilder fail if any of the middle GEP indices are non-constant.

This is a fix for a bug in r317947. We were supposed to check that all the indices are are constant 0, but instead we're only make sure that indices that are constant are 0. Non-constant indices are being ignored.

llvm-svn: 317950