aco: only break SMEM clauses if XNACK is enabled (mostly APUs)

aco: only break SMEM clauses if XNACK is enabled (mostly APUs)

According to LLVM, it seems only required for APUs like RAVEN, but
we still ensure that SMEM stores are in their own clause.

pipeline-db (VEGA10):
Totals from affected shaders:
SGPRS: 1775364 -> 1775364 (0.00 %)
VGPRS: 1287176 -> 1287176 (0.00 %)
Spilled SGPRs: 725 -> 725 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Code Size: 65386620 -> 65107460 (-0.43 %) bytes
Max Waves: 287099 -> 287099 (0.00 %)

pipeline-db (POLARIS10):
Totals from affected shaders:
SGPRS: 1797743 -> 1797743 (0.00 %)
VGPRS: 1271108 -> 1271108 (0.00 %)
Spilled SGPRs: 730 -> 730 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Code Size: 64046244 -> 63782324 (-0.41 %) bytes
Max Waves: 254875 -> 254875 (0.00 %)

This only affects GFX6-GFX9 chips because the compiler uses a
different pass for GFX10.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4349>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4349>