LLIR scheduler + amdgcnas + RA-hints, rebased onto upstream main (63a5e1f0e).
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-v0.3.