[X86] Fix a bug in getMemOpBaseRegImmOfs

Fix a crash in `getMemOpBaseRegImmOfs` that happens if the base of
`MemOp` is a frame index memory operand.  The fix is to have
`getMemOpBaseRegImmOfs` bail out in such cases.  We can possibly be more
clever here, if needed.

llvm-svn: 259456