[X86] Change an llvm_unreachable to a report_fatal_error so the optimizer will...

[X86] Change an llvm_unreachable to a report_fatal_error so the optimizer will stop making us reach the other report_fatal_error in this function.

There's a conditional report_fatal_error just above this llvm_unreachable. The optimizer when seeing the unreachable removes the conditional and just makes any other error trigger the existing report_fatal_error.

llvm-svn: 343428