X86: FrameIndex addressing modes do have a base register.

When selecting the DAG (add (WrapperRIP ...), (FrameIndex ...)), X86 code had
spotted the FrameIndex possibility and was working out whether it could fold
the WrapperRIP into this.

The test for forming a %rip version is notionally whether we already have a
base or index register (%rip precludes both), but we were forgetting to account
for the register that would be inserted later to access the frame.

rdar://problem/15024520