Merging r343428:

Merging r343428:

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r343428 | ctopper | 2018-09-30 16:43:30 -0700 (Sun, 30 Sep 2018) | 3 lines

[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.
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_70@344805 91177308-0d34-0410-b5e6-96231b3b80d8