[mlir][Transforms] Fix use-after-free when accessing replaced block args (#83646)

This commit fixes a bug in a dialect conversion. Currently, when a block
is replaced via a signature conversion, the block is erased during the
"commit" phase. This is problematic because the block arguments may
still be referenced internal data structures of the dialect conversion
(`mapping`). Blocks should be treated same as ops: they should be erased
during the "cleanup" phase.

Note: The test case fails without this fix when running with ASAN, but
may pass when running without ASAN.