pan/bi: Use padding bytes for checking whether to stop disassembly

pan/bi: Use padding bytes for checking whether to stop disassembly

Both Panfrost and the DDK add padding zero bytes to the end of
shaders, so we can use this instead of the end-of-shader clause for
checking whether to stop disassembling.

Shaders can have end-of-shader clauses partway through; these shaders
will now be completely disassembled instead of cut off at the first
end-of-shader clause.

A tag byte of zero is an invalid encoding, so unlike the previous
version of this test only check the first word.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12153>