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

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

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.

Reviewed-by: Rob Clark <robdclark@chromium.org>