[llvm] [NFC][AMDGPU] Add debug print to `AMDGPULowerVGPREncoding.cpp` (PR #185331)

[Tim Renouf via llvm-commits]

================
@@ -98,6 +99,21 @@ class AMDGPULowerVGPREncoding {
       return V;
     }
 
+    void print(raw_ostream &OS) const {
+      static const char *FieldNames[] = {"src0", "src1", "src2", "dst"};
----------------
trenouf wrote:

`OpNum` is 7, so this array needs 7 entries. Or change the loop to enumerate no more than the number of entries in this array. I get this warning:

```
llvm-project/llvm/lib/Target/AMDGPU/AMDGPULowerVGPREncoding.cpp:171: warning: iteration 4 invokes undefined behavior [-Waggressive-loop-optimizations]
```

https://github.com/llvm/llvm-project/pull/185331