aco: Fix 8 and 16-bit addition reductions on GFX6-7
On GFX6-7, the 8 and 16-bit integer add reductions use the 32-bit v_add instruction, which clobbers the VCC register. But we forgot to mark it as such, which lead to test failures.
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On GFX6-7, the 8 and 16-bit integer add reductions use the 32-bit v_add instruction, which clobbers the VCC register. But we forgot to mark it as such, which lead to test failures.