r300/compiler: Move declaration before code.

r300/compiler: Move declaration before code.

Fixes these GCC warnings on linux-x86 build.
radeon_dataflow_deadcode.c: In function ‘push_branch’:
radeon_dataflow_deadcode.c:112: warning: ISO C90 forbids mixed declarations and code
radeon_dataflow_deadcode.c: In function ‘update_instruction’:
radeon_dataflow_deadcode.c:183: warning: ISO C90 forbids mixed declarations and code
radeon_dataflow_deadcode.c: In function ‘rc_dataflow_deadcode’:
radeon_dataflow_deadcode.c:352: warning: ISO C90 forbids mixed declarations and code
radeon_dataflow_deadcode.c:379: warning: ISO C90 forbids mixed declarations and code