[mlir][Transforms] OneToNTypeConversion.cpp: Fix invalid IR (llvm#77922)

`buildUnrealizedCast` used to generate invalid
`builtin.unrealized_conversion_cast` ops with zero results. This commit
fixes
`test/Conversion/OneToNTypeConversion/one-to-n-type-conversion.mlir`
when running with `MLIR_ENABLE_EXPENSIVE_PATTERN_API_CHECKS`.

```
  * Pattern (anonymous namespace)::ConvertMakeTupleOp : 'test.make_tuple -> ()' {
Trying to match "(anonymous namespace)::ConvertMakeTupleOp"

[...]

"(anonymous namespace)::ConvertMakeTupleOp" result 1
  } -> success : pattern applied successfully
// *** IR Dump After Pattern Application ***
mlir-asm-printer: Verifying operation: func.func
'builtin.unrealized_conversion_cast' op expected at least one result for cast operation
mlir-asm-printer: 'func.func' failed to verify and will be printed in generic form
"func.func"() <{function_type = (i1, i2) -> (i1, i2), sym_name = "pack_unpack"}> ({
^bb0(%arg0: i1, %arg1: i2):
  %0 = "test.make_tuple"() : () -> tuple<>
  "builtin.unrealized_conversion_cast"(%0) {"__one-to-n-type-conversion_cast-kind__" = "target"} : (tuple<>) -> ()

[...]

}) : () -> ()

within split at /usr/local/google/home/springerm/mlir_public/llvm-project/mlir/test/Conversion/OneToNTypeConversion/one-to-n-type-conversion.mlir:1 offset :20:8: error: 'builtin.unrealized_conversion_cast' op expected at least one result for cast operation
  %0 = "test.make_tuple"() : () -> tuple<>
       ^
within split at /usr/local/google/home/springerm/mlir_public/llvm-project/mlir/test/Conversion/OneToNTypeConversion/one-to-n-type-conversion.mlir:1 offset :20:8: note: see current operation: "builtin.unrealized_conversion_cast"(%0) {"__one-to-n-type-conversion_cast-kind__" = "target"} : (tuple<>) -> ()
LLVM ERROR: IR failed to verify after pattern application
```

Author: matthias-springer

mlir/lib/Transforms/Utils/OneToNTypeConversion.cpp

@@ -113,6 +113,11 @@ static const char *const castKindAttrName =
 /// result types. Returns the result values of the cast.
 static ValueRange buildUnrealizedCast(OpBuilder &builder, TypeRange resultTypes,
                                       ValueRange inputs, CastKind kind) {
+  // Special case: 1-to-N conversion with N = 0. No need to build an
+  // UnrealizedConversionCastOp because the op will always be dead.
+  if (resultTypes.empty())
+    return ValueRange();
+
   // Create cast.
   Location loc = builder.getUnknownLoc();
   if (!inputs.empty())