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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343428 91177308-0d34-0410-b5e6-96231b3b80d8