broadcom/compiler: fix assert that current instruction must be in current block
This was not considering the possibility that the driver has called nir_before_block() or nir_after_block() to update the cursor, in which case the cursor link points to the instruction list header and not to an actual instruction.
Fixes incorrect debug-assert crash in: dEQP-VK.graphicsfuzz.cov-increment-vector-component-with-matrix-copy
Fixes: 265515fa ("broadcom/compiler: check instruction belongs to current block")