[mlir] Fix body-less async.execute printing

[ezhulenev]

Execute without a body doesn't make much sense, maybe check it in verifier? Currently it will also fail because of op.body() access

[csigg]

An op should be printable even if it's not valid. In particular, running conversions where async.execute bodies are moved to clones should not crash with '-debug'. The op.body() access is fine, the region always exists but may be empty.

mlir/lib/Dialect/Async/IR/Async.cpp

static void print(OpAsmPrinter &p, ExecuteOp op) {
 
   // [%tokens,...]
   if (!op.dependencies().empty())
     p << " [" << op.dependencies() << "]";
 
   // (%value as %unwrapped: !async.value<!arg.type>, ...)
   if (!op.operands().empty()) {
     p << " (";
+    Block *entry = op.body().empty() ? nullptr : &op.body().front();
     llvm::interleaveComma(op.operands(), p, [&, n = 0](Value operand) mutable {
-      p << operand << " as " << op.body().front().getArgument(n++) << ": "
-        << operand.getType();
+      Value argument = entry ? entry->getArgument(n++) : Value();
+      p << operand << " as " << argument << ": " << operand.getType();
     });
     p << ")";
   }
 
   // -> (!async.value<!return.type>, ...)
   p.printOptionalArrowTypeList(llvm::drop_begin(op.getResultTypes()));
   p.printOptionalAttrDictWithKeyword(op->getAttrs(),
                                      {kOperandSegmentSizesAttr});