[llvm] [NFC][AMDGPU] Add tests for SCC live into a frame index user (PR #225196)

[Domenic Nutile via llvm-commits]

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@@ -166,3 +166,156 @@ body:             |
 
     S_ENDPGM 0, implicit $vcc
 ...
+
+---
+# SCC is live *into* the scalar frame index user: S_CMP_EQ_U32 produces it and
+# S_CSELECT_B32 reads it. With no SGPR free to hold the V_READFIRSTLANE_B32
+# result, scavenging one emergency-spills, and an SGPR spill flips EXEC with
+# S_NOT_B64. Those EXEC flips land between the S_CMP_EQ_U32 and the
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saxlungs wrote:

Yes, I missed that one originally because it actually errors on the pre-fix version. I've added that in now

https://github.com/llvm/llvm-project/pull/225196