[llvm] AMDGPU: Migrate more tests away from undef (PR #131314)

[Matt Arsenault via llvm-commits]

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@@ -538,7 +538,8 @@ if.then:                                          ; preds = %entry
   ret void
 
 if.then9:                                         ; preds = %entry
-  br i1 undef, label %sw.bb18, label %sw.bb
+  %undef = freeze i1 poison
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arsenm wrote:

This is mostly trying to avoid perturbing codegen. Ideally would avoid using br on poison directly, but that breaks the direct branch on constant property some of these tests are relying on 

https://github.com/llvm/llvm-project/pull/131314