Fix ConditionalSpeculatability implementation for DynamicReshape (#2261)

Both the input and the shape must be fully known statically. Whether the
output is dynamic or not does not matter.

Indeed, consider e.g.:

```
func.func @foo(%arg0: tensor<?x?xf64>) {
    %constant_shape = stablehlo.constant dense<[2, 3]> : tensor<2xi32>
    %0 = stablehlo.dynamic_reshape %arg0, %constant_shape : (tensor<?x?xf64>, tensor<2xi32>) -> tensor<?x?xf64>
    return
}
```

The input is dynamic, so it could turn out to have e.g. 10 elements
instead of the expected 6. Similarly, if the shape is unknown:

```
func.func @foo(%arg0: tensor<2x3xf64>, %unknown_shape: tensor<2xi32>) {
    %0 = stablehlo.dynamic_reshape %arg0, %unknown_shape : (tensor<2x3xf64>, tensor<2xi32>) -> tensor<?x?xf64>
    return
}
```

Again, the shape could turn out to be e.g. `[2, 5]` at runtime and so
the reshape's behavior would be undefined.

stablehlo/dialect/StablehloOps.cpp

@@ -1510,18 +1510,10 @@ LogicalResult DynamicReshapeOp::reifyReturnTypeShapes(
 }
 
 mlir::Speculation::Speculatability DynamicReshapeOp::getSpeculatability() {
-  // If the output type's shape is fully dynamic, there is no expectation
-  // for the shape so the op is speculatable.
-  if (llvm::all_of(llvm::seq(getType().getRank()),
-                   [this](int64_t i) { return getType().isDynamicDim(i); }))
-    return mlir::Speculation::Speculatable;
-
   // If the input is static and the shape operand is constant, the output
   // shape can be inferred and any mismatch will be caught statically.
-  // If any dimension in the input is dynamic, the number of elements may
-  // disagree with either the output.
-  // If the shape operand is not constant, it could disagree with the output,
-  // which has at least 1 static dimension at this point in the function.
+  // If any dimension in the input is dynamic, or if the shape is not known,
+  // the number of elements may disagree at runtime.
   if (getOperand().getType().hasStaticShape() &&
       matchPattern(getOutputShape(), m_Constant()))
     return mlir::Speculation::Speculatable;

stablehlo/tests/ops_speculatability.mlir

@@ -1750,13 +1750,13 @@ func.func @dynamic_reshape(
   %2 = stablehlo.dynamic_reshape %dynamic_arg, %constant_shape : (tensor<?x?xf64>, tensor<2xi32>) -> tensor<5x4xf64>
   "hlo_test_speculatability.is_not_speculatable"(%2) : (tensor<5x4xf64>) -> ()
   %3 = stablehlo.dynamic_reshape %dynamic_arg, %constant_shape : (tensor<?x?xf64>, tensor<2xi32>) -> tensor<?x?xf64>
-  "hlo_test_speculatability.is_speculatable"(%3) : (tensor<?x?xf64>) -> ()
+  "hlo_test_speculatability.is_not_speculatable"(%3) : (tensor<?x?xf64>) -> ()
 
   // Unknown shape
   %4 = stablehlo.dynamic_reshape %static_arg, %unknown_shape : (tensor<4x5xf64>, tensor<2xi32>) -> tensor<5x4xf64>
   "hlo_test_speculatability.is_not_speculatable"(%4) : (tensor<5x4xf64>) -> ()
   %5 = stablehlo.dynamic_reshape %static_arg, %unknown_shape : (tensor<4x5xf64>, tensor<2xi32>) -> tensor<?x?xf64>
-  "hlo_test_speculatability.is_speculatable"(%5) : (tensor<?x?xf64>) -> ()
+  "hlo_test_speculatability.is_not_speculatable"(%5) : (tensor<?x?xf64>) -> ()
 
   return
 }