LLIR scheduler + amdgcnas + RA-hints, rebased onto upstream main. Adds
MMRA-fence-aware barrier handling (keeps release/acquire fences glued to the
s.barrier) and decouples the LLVM misched-disable from the env var (driven off
whether the LLIR scheduler actually ran). Companion to the gfx950-gluon-tutorials
repo at tag gfx950-tutorial-v0.2.