[llvm] [AMDGPU] Enable saving SHARED_BASE to VCC (PR #163244)

[Matt Arsenault via llvm-commits]

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@@ -58,6 +58,15 @@ body:             |
     $sgpr0_sgpr1 = COPY $src_shared_base
 ...
 
+---
+name: src_shared_base_to_vcc
+body:             |
+  bb.0:
+    ; GFX9-LABEL: name: src_shared_base_to_vcc
+    ; GFX9: $vcc = S_MOV_B64 $src_shared_base
+    $vcc = COPY $src_shared_base
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arsenm wrote:

ping. We do not structurally prevent writing to constant registers and it is encodable 

https://github.com/llvm/llvm-project/pull/163244