[mlir][DialectConversion] Do not prematurely drop unused cast operations

The rewrite logic has an optimization to drop a cast operation after
rewriting block arguments if the cast operation has no users. This is
unsafe as there might be a pending rewrite that replaced the cast operation
itself and hence would trigger a second free.

Instead, do not remove the casts and leave it up to a later canonicalization
to do so.

Differential Revision: https://reviews.llvm.org/D92184

Author: Stephan Herhut

mlir/lib/Transforms/Utils/DialectConversion.cpp

@@ -364,11 +364,6 @@ void ArgConverter::applyRewrites(ConversionValueMapping &mapping) {
       // If the argument is still used, replace it with the generated cast.
       if (!origArg.use_empty())
         origArg.replaceAllUsesWith(mapping.lookupOrDefault(castValue));
-
-      // If all users of the cast were removed, we can drop it. Otherwise, keep
-      // the operation alive and let the user handle any remaining usages.
-      if (castValue.use_empty() && castValue.getDefiningOp())
-        castValue.getDefiningOp()->erase();
     }
   }
 }

mlir/test/Transforms/test-legalizer.mlir

@@ -36,8 +36,9 @@ func @remap_call_1_to_1(%arg0: i64) {
 
 // CHECK-LABEL: func @remap_input_1_to_N({{.*}}f16, {{.*}}f16)
 func @remap_input_1_to_N(%arg0: f32) -> f32 {
- // CHECK-NEXT: "test.return"{{.*}} : (f16, f16) -> ()
- "test.return"(%arg0) : (f32) -> ()
+  // CHECK-NEXT: [[CAST:%.*]] = "test.cast"(%arg0, %arg1) : (f16, f16) -> f32
+  // CHECK-NEXT: "test.return"{{.*}} : (f16, f16) -> ()
+  "test.return"(%arg0) : (f32) -> ()
 }
 
 // CHECK-LABEL: func @remap_input_1_to_N_remaining_use(%arg0: f16, %arg1: f16)