[mlir][Transforms] Dialect conversion: Fix bug in computeNecessaryMaterializations (#104630)

There was a typo in the code path that removes unnecessary
materializations.

Before: Update `opResult` (result of an op different from `user`) in
mapping and remove `user`.
```
replaceMaterialization(rewriterImpl, opResult, inputOperands,
                       inverseMapping);
necessaryMaterializations.remove(materializationOps.lookup(user));
```

After: Update `user->getResults()` in mapping and remove `user`.
```
replaceMaterialization(rewriterImpl, user->getResults(), inputOperands,
                       inverseMapping);
necessaryMaterializations.remove(materializationOps.lookup(user));
```

Author: matthias-springer

mlir/lib/Transforms/Utils/DialectConversion.cpp

@@ -2680,7 +2680,7 @@ static void computeNecessaryMaterializations(
       if (!castOp)
         continue;
       if (castOp->getResultTypes() == inputOperands.getTypes()) {
-        replaceMaterialization(rewriterImpl, opResult, inputOperands,
+        replaceMaterialization(rewriterImpl, user->getResults(), inputOperands,
                                inverseMapping);
         necessaryMaterializations.remove(materializationOps.lookup(user));
       }

mlir/test/Transforms/test-legalize-type-conversion.mlir

@@ -99,6 +99,7 @@ func.func @test_block_argument_not_converted() {
 func.func @test_signature_conversion_no_converter() {
   "test.signature_conversion_no_converter"() ({
   // expected-error@below {{failed to legalize unresolved materialization from ('f64') to 'f32' that remained live after conversion}}
+  // expected-note@below {{see existing live user here}}
   ^bb0(%arg0: f32):
     "test.type_consumer"(%arg0) : (f32) -> ()
     "test.return"(%arg0) : (f32) -> ()