Similar to `OpBuilder::clone`, operation/block insertion notifications
should be sent when cloning the contents of a region. E.g., this is to
ensure that the newly created operations are put on the worklist of the
greedy pattern rewriter driver.
Also move `cloneRegionBefore` from `RewriterBase` to `OpBuilder`. It
only creates new IR, so it should be part of the builder API (like
`clone(Operation &)`). The function does not have to be virtual. Now
that notifications are properly sent, the override in the dialect
conversion is no longer needed.
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