[MLIR] Harmonize the behavior of the folding API functions (#88508)

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.

The added test contains a simplified version of a breakage we observed
downstream.