[MLIR] Harmonize the behavior of the folding API functions

This commit changes `OpBuilder::tryFold` to behave more similarly to
`Operation::fold`. Concretely, this ensures that even an in-place fold
returns `success`. This is necessary to fix a bug in the dialect
conversion that occurred when an in-place folding made an operation
legal. The dialect conversion infrastructure did not check if the result
of an in-place folding legalized the operation and just went ahead and
tried to apply pattern anyways.