nir/cf: add split_block_cursor()
nir/cf: add split_block_cursor()

This is a helper that will be shared between the new control flow
insertion and modification code.

Signed-off-by: Connor Abbott <99c5849d46b9d5d9781f7baa7735ff66305c63a7@intel.com>
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler