ac/nir: fix atomic compare-and-swap

The LLVM instruction returns { i32, i1 }, where the i1 indicates success.
We're only interested in the first part, which is the loaded value.

Fixes dEQP-GLES31.functional.compute.shared_var.atomic.compswap.*

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/amd/common/ac_nir_to_llvm.c

@@ -2631,6 +2631,7 @@ static LLVMValueRef visit_var_atomic(struct ac_nir_context *ctx,
 						LLVMAtomicOrderingSequentiallyConsistent,
 						LLVMAtomicOrderingSequentiallyConsistent,
 						false);
+		result = LLVMBuildExtractValue(ctx->ac.builder, result, 0, "");
 	} else {
 		LLVMAtomicRMWBinOp op;
 		switch (instr->intrinsic) {