nir/cf: add a cursor structure

For now, it allows us to refactor the control flow insertion API's so
that there's a single entrypoint (with some wrappers). More importantly,
it will allow us to reduce the combinatorial explosion in the extract
function. There, we need to specify two points to extract, which may be
at the beginning of a block, the end of a block, or in the middle of a
block. And then there are various wrappers based off of that (before a
control flow node, before a control flow list, etc.). Rather than having
9 different functions, we can have one function and push the actual
logic of determining which variant to use down to the split function,
which will be shared with nir_cf_node_insert().

In the future, we may want to make the instruction insertion API's as
well as the builder use this, but that's a future cleanup.

Signed-off-by: Connor Abbott <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>

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