X86: loosen an overly aggressive MachO assertion

We would assert that the FP setup CFI used esp/rsp always.  This held up in
practice when the code was generated from IR.  However, with the integrated
assembler, it is possible to have the input be user specified assembly.  In such
a case, we cannot assume that the function implementation has a compact unwind
representation.  Loosen the assertion into a check and bail if we cannot
represent the frame pointer in the compact unwinding.

Addresses PR30453!

llvm-svn: 281986