[llvm] [X86] Handle BSF/BSR "zero-input pass through" behaviour (PR #123623)

[Phoebe Wang via llvm-commits]

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@@ -15,12 +15,12 @@ bsf   %rax, %rcx
 
 # CHECK:      Iterations:        100
 # CHECK-NEXT: Instructions:      400
-# CHECK-NEXT: Total Cycles:      655
+# CHECK-NEXT: Total Cycles:      663
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phoebewang wrote:

I think it makes sense somehow. The dependency is increased indeed. Fortunately BSF doesn't have partial dependency issue.

https://github.com/llvm/llvm-project/pull/123623