[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.

0 files changed, 0 insertions, 0 deletions