[mlir][Transforms] Fix compile time regression in dialect conversion (#83023)

The dialect conversion does not directly erase ops that are
replaced/erased with a rewriter. Instead, the op stays in place and is
erased at the end if the dialect conversion succeeds. However, ops that
were replaced/erased are ignored from that point on.

#81757 introduced a compile time regression that made the check whether
an op is ignored or not more expensive. Whether an op is ignored or not
is queried many times throughout a dialect conversion, so the check must
be fast.

After this change, replaced ops are stored in the `ignoredOps` set. This
also simplifies the dialect conversion a bit.