[mlir][Transforms] Simplify ConversionPatternRewriter::replaceOp implementation (#158075)

Move the logic for building "out-of-thin-air" source materializations
during op replacements from `replaceOp` to
`findOrBuildReplacementValue`. That function already builds source
materializations and can handle the case where an op result is dropped.

This commit is in preparation of turning `replaceOp` into a non-virtual
function. (It is sufficient for `replaceAllUsesWith` and `eraseOp` to be
virtual.)

Author: matthias-springer

mlir/lib/Transforms/Utils/DialectConversion.cpp

@@ -1618,6 +1618,8 @@ Block *ConversionPatternRewriterImpl::applySignatureConversion(
     if (!inputMap) {
       // This block argument was dropped and no replacement value was provided.
       // Materialize a replacement value "out of thin air".
+      // Note: Materialization must be built here because we cannot find a
+      // valid insertion point in the new block. (Will point to the old block.)
       Value mat =
           buildUnresolvedMaterialization(
               MaterializationKind::Source,
@@ -1725,29 +1727,29 @@ Value ConversionPatternRewriterImpl::findOrBuildReplacementValue(
   // (regardless of the type) and build a source materialization to the
   // original type.
   repl = lookupOrNull(value);
+
+  // Compute the insertion point of the materialization.
+  OpBuilder::InsertPoint ip;
   if (repl.empty()) {
-    // No replacement value is registered in the mapping. This means that the
-    // value is dropped and no longer needed. (If the value were still needed,
-    // a source materialization producing a replacement value "out of thin air"
-    // would have already been created during `replaceOp` or
-    // `applySignatureConversion`.)
-    return Value();
+    // The source materialization has no inputs. Insert it right before the
+    // value that it is replacing.
+    ip = computeInsertPoint(value);
+  } else {
+    // Compute the "earliest" insertion point at which all values in `repl` are
+    // defined. It is important to emit the materialization at that location
+    // because the same materialization may be reused in a different context.
+    // (That's because materializations are cached in the conversion value
+    // mapping.) The insertion point of the materialization must be valid for
+    // all future users that may be created later in the conversion process.
+    ip = computeInsertPoint(repl);
   }
-
-  // Note: `computeInsertPoint` computes the "earliest" insertion point at
-  // which all values in `repl` are defined. It is important to emit the
-  // materialization at that location because the same materialization may be
-  // reused in a different context. (That's because materializations are cached
-  // in the conversion value mapping.) The insertion point of the
-  // materialization must be valid for all future users that may be created
-  // later in the conversion process.
-  Value castValue =
-      buildUnresolvedMaterialization(MaterializationKind::Source,
-                                     computeInsertPoint(repl), value.getLoc(),
-                                     /*valuesToMap=*/repl, /*inputs=*/repl,
-                                     /*outputTypes=*/value.getType(),
-                                     /*originalType=*/Type(), converter)
-          .front();
+  Value castValue = buildUnresolvedMaterialization(
+                        MaterializationKind::Source, ip, value.getLoc(),
+                        /*valuesToMap=*/repl, /*inputs=*/repl,
+                        /*outputTypes=*/value.getType(),
+                        /*originalType=*/Type(), converter,
+                        /*isPureTypeConversion=*/!repl.empty())
+                        .front();
   return castValue;
 }
 
@@ -1897,21 +1899,8 @@ void ConversionPatternRewriterImpl::replaceOp(
   }
 
   // Create mappings for each of the new result values.
-  for (auto [repl, result] : llvm::zip_equal(newValues, op->getResults())) {
-    if (repl.empty()) {
-      // This result was dropped and no replacement value was provided.
-      // Materialize a replacement value "out of thin air".
-      buildUnresolvedMaterialization(
-          MaterializationKind::Source, computeInsertPoint(result),
-          result.getLoc(), /*valuesToMap=*/{result}, /*inputs=*/ValueRange(),
-          /*outputTypes=*/result.getType(), /*originalType=*/Type(),
-          currentTypeConverter, /*isPureTypeConversion=*/false);
-      continue;
-    }
-
-    // Remap result to replacement value.
+  for (auto [repl, result] : llvm::zip_equal(newValues, op->getResults()))
     mapping.map(static_cast<Value>(result), std::move(repl));
-  }
 
   appendRewrite<ReplaceOperationRewrite>(op, currentTypeConverter);
   // Mark this operation and all nested ops as replaced.