freedreno: Fix invalid read when a block has no instructions.

Emma Anholt requested to merge anholt/mesa:fd-block-fix into master

We can't deref list_(first/last)_entries unless we know we have at least one. Instead, just use our IP we've been tracking as we go to set up the start ip, and fill in the end IP as we walk instructions.

Fixes a complaint in valgrind on dEQP-GLES3.functional.transform_feedback.* which sometimes has an empty main (non-END) block when the VS inputs are just directly mapped to outputs without any ALU ops.

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