radeonsi: strengthen emit_optimization_barrier

radeonsi: strengthen emit_optimization_barrier

LLVM will lift inline assembly out of if-else-blocks if both paths have
the same inline assembly. Prevent this by adding an irrelevant unique
text to the assembly.

This requires the LLVM assembly parser to be initialized.

Furthermore, allow forcing subsequent computations to happen after the
optimization barrier by defining a data dependency.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>