[MLIR] Fix block label parsing bug

Fix bug in `Block` label parsing:
https://github.com/llvm/llvm-project/issues/54343

The `parseOptionalBlockArgList` method was doing the wrong thing
(contrary to its doc comment) and its calling context was also
incorrect. This led to a parse failure for something like "^bb0()".

Fixes #54343

Differential Revision: https://reviews.llvm.org/D121503