[Stablehlo] fix compareOp with scalar's lowering (#3518)

* use lhs tensor's element type as compute type when rhs is scalar.
* previously `a != 1.0`(a is a fp32 tensor) will lowering to `%6 =
stablehlo.compare EQ, %4, %5, FLOAT : (tensor<2x5xf64>, tensor<2x5xf64>)
-> tensor<2x5xi1>`
* now it will lowering to `%6 = stablehlo.compare EQ, %4, %5, FLOAT :
(tensor<2x5xf32>, tensor<2x5xf32>) -> tensor<2x5xi1>`

lib/Conversion/TorchToStablehlo/Basic.cpp

@@ -517,6 +517,8 @@ public:
     if (!rhsTy) {
       rhs = hlo::scalarToStablehloTensor(rewriter, op, adaptor.getOther(),
                                          rhs.getType());
+      // use lhs's element type as compute type
+      rhs = hlo::promoteType(rewriter, op.getLoc(), rhs, lhsTy);
       rhsTy = dyn_cast<RankedTensorType>(rhs.getType());
     }