ac,ac/nir: use a better sync scope for shared atomics

ac,ac/nir: use a better sync scope for shared atomics

https://reviews.llvm.org/rL356946 (present in LLVM 9 and later) changed
the meaning of the "system" sync scope, making it no longer restricted to
the memory operation's address space. So a single address space sync scope
is needed for shared atomic operations (such as "system-one-as" or
"workgroup-one-as") otherwise buffer_wbinvl1 and s_waitcnt instructions
can be created at each shared atomic operation.

This mostly reimplements LLVMBuildAtomicRMW and LLVMBuildAtomicCmpXchg
to allow for more sync scopes and uses the new functions in ac->nir with
the "workgroup-one-as" or "workgroup" sync scopes.

      F1 2017 (4K, Ultra High settings, TAA), avg FPS : 59 -> 59.67 (+1.14%)
     Strange Brigade (4K, ~highest settings), avg FPS : 51.5 -> 51.6 (+0.19%)
RotTR/mountain (4K, VeryHigh settings, FXAA), avg FPS : 57.2 -> 57.2 (+0.0%)
    RotTR/tomb (4K, VeryHigh settings, FXAA), avg FPS : 42.5 -> 43.0 (+1.17%)
  RotTR/valley (4K, VeryHigh settings, FXAA), avg FPS : 40.7 -> 41.6 (+2.21%)
                         Warhammer II/fallen, avg FPS : 31.63 -> 31.83 (+0.63%)
                         Warhammer II/skaven, avg FPS : 37.77 -> 38.07 (+0.79%)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>