[SelectionDAG] Use report_fatal_error instead of llvm_unreachable in some code that can be reached if targets don't configure things correctly.

For example, this is currently reachable by X86 if you use a masked store intrinsic with a v1iX type.

Using a fatal error seems like a better user experience if someone were to encounter this on a release build. There are several other similar places that have been converted from unreachable to fatal error previously.

llvm-svn: 318379

llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp

@@ -461,7 +461,8 @@ bool DAGTypeLegalizer::ScalarizeVectorOperand(SDNode *N, unsigned OpNo) {
       N->dump(&DAG);
       dbgs() << "\n";
 #endif
-      llvm_unreachable("Do not know how to scalarize this operator's operand!");
+      report_fatal_error("Do not know how to scalarize this operator's "
+                         "operand!\n");
     case ISD::BITCAST:
       Res = ScalarizeVecOp_BITCAST(N);
       break;