r600g/sb: separate bytecode decoding and parsing

r600g/sb: separate bytecode decoding and parsing

Parsing and ir construction is required for optimization only,
it's unnecessary if we only need to print shader dump.
This should make new disassembler more tolerant to any new
features in the bytecode.

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>