Add the LLVM IR unreachable instruction to the LLVMIR dialect.

http://llvm.org/docs/LangRef.html#unreachable-instruction

Closes tensorflow/mlir#64

PiperOrigin-RevId: 262301557

mlir/include/mlir/LLVMIR/LLVMOps.td

@@ -300,6 +300,11 @@ def LLVM_ReturnOp : LLVM_TerminatorOp<"return", []> {
   let parser = [{ return parseReturnOp(parser, result); }];
   let printer = [{ printReturnOp(p, *this); }];
 }
+def LLVM_UnreachableOp : LLVM_TerminatorOp<"unreachable", []> {
+  string llvmBuilder = [{ builder.CreateUnreachable(); }];
+  let parser = [{ return success(); }];
+  let printer = [{ *p << getOperationName(); }];
+}
 
 // Pseudo-operations (do not appear in LLVM IR but necessary for the dialect to
 // work correctly).

mlir/test/Target/llvmir.mlir

@@ -825,3 +825,7 @@ func @stringconstant() -> !llvm<"i8*"> {
   llvm.return %1 : !llvm<"i8*">
 }
 
+func @noreach() {
+// CHECK:    unreachable
+  llvm.unreachable
+}