aco: explicitly mark end blocks for exports

aco: explicitly mark end blocks for exports

For GS copy shaders, whether we want to do exports is conditional. By
explicitly marking the end blocks, we can mark an IF's then branch as an
export block and ensure that's where the assembler inserts null exports.

v6: only fixup exports in the end block, like before
v8: simplify some code

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2421>