[mlir][Transforms][NFC] Dialect Conversion: Remove redundant lookupOrDefault (#110370)

Remove a redundant `lookupOrDefault` that has no effect.

When no type is passed to `lookupOrDefault`, that function returns the
furthest mapped value (by following the mapping iteratively). If there
is no mapped value with the desired type, then the function also returns
the furthest mapped value.

The value that was passed to the redundant `lookupOrDefault` was
produced by this code:
```
Value newOperand = mapping.lookupOrDefault(operand, desiredType);
```

There are 2 possible cases:
- Case 1: There is no mapping to `desiredType`. Then `newOperand` is the
furthest mapped value.
- Case 2: There is a mapping to `desiredType`. Then the type of
`newOperand` is `desiredType` and the "if" branch that encloses the
redundant `lookupOrDefault` is not executed at all.

Also improve the documentation of
`ConversionValueMapping::lookupOrDefault` and simplify the
implementation a bit.

Author: matthias-springer

mlir/lib/Transforms/Utils/DialectConversion.cpp

@@ -71,10 +71,16 @@ namespace {
 /// This class wraps a IRMapping to provide recursive lookup
 /// functionality, i.e. we will traverse if the mapped value also has a mapping.
 struct ConversionValueMapping {
-  /// Lookup a mapped value within the map. If a mapping for the provided value
-  /// does not exist then return the provided value. If `desiredType` is
-  /// non-null, returns the most recently mapped value with that type. If an
-  /// operand of that type does not exist, defaults to normal behavior.
+  /// Lookup the most recently mapped value with the desired type in the
+  /// mapping.
+  ///
+  /// Special cases:
+  /// - If the desired type is "null", simply return the most recently mapped
+  ///   value.
+  /// - If there is no mapping to the desired type, also return the most
+  ///   recently mapped value.
+  /// - If there is no mapping for the given value at all, return the given
+  ///   value.
   Value lookupOrDefault(Value from, Type desiredType = nullptr) const;
 
   /// Lookup a mapped value within the map, or return null if a mapping does not
@@ -115,19 +121,11 @@ struct ConversionValueMapping {
 
 Value ConversionValueMapping::lookupOrDefault(Value from,
                                               Type desiredType) const {
-  // If there was no desired type, simply find the leaf value.
-  if (!desiredType) {
-    // If this value had a valid mapping, unmap that value as well in the case
-    // that it was also replaced.
-    while (auto mappedValue = mapping.lookupOrNull(from))
-      from = mappedValue;
-    return from;
-  }
-
-  // Otherwise, try to find the deepest value that has the desired type.
+  // Try to find the deepest value that has the desired type. If there is no
+  // such value, simply return the deepest value.
   Value desiredValue;
   do {
-    if (from.getType() == desiredType)
+    if (!desiredType || from.getType() == desiredType)
       desiredValue = from;
 
     Value mappedValue = mapping.lookupOrNull(from);
@@ -1136,7 +1134,7 @@ LogicalResult ConversionPatternRewriterImpl::remapValues(
           MaterializationKind::Target, computeInsertPoint(newOperand),
           operandLoc, /*inputs=*/newOperand, /*outputType=*/desiredType,
           currentTypeConverter);
-      mapping.map(mapping.lookupOrDefault(newOperand), castValue);
+      mapping.map(newOperand, castValue);
       newOperand = castValue;
     }
     remapped.push_back(newOperand);