nir_to_tgsi: Require the block index to always be populated
In some cases like when using NIR_DEBUG=serialize, impl->num_blocks is 0
which leads to assertions error in the blocklist. Make sure to require the
num_blocks to be populated.
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In some cases like when using NIR_DEBUG=serialize, impl->num_blocks is 0
which leads to assertions error in the blocklist. Make sure to require the
num_blocks to be populated.