[llvm] [WIP][AMDGPU][Attributor] Make `AAAMDFlatWorkGroupSize` honor existing attribute (PR #114357)

[Matt Arsenault via llvm-commits]

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@@ -168,7 +168,15 @@ class AMDGPUInformationCache : public InformationCache {
     return ST.supportsGetDoorbellID();
   }
 
-  std::pair<unsigned, unsigned> getFlatWorkGroupSizes(const Function &F) {
+  std::optional<std::pair<unsigned, unsigned>>
+  getFlatWorkGroupSizeAttr(const Function &F) const {
+    Attribute A = F.getFnAttribute("amdgpu-flat-work-group-size");
+    if (!A.isStringAttribute())
+      return std::nullopt;
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arsenm wrote:

The presence or absence of an attribute should not impart meaning. A function explicitly marked with the default value should be the same as one with no attribute.

This will also miss applying the calling convention based default range 

https://github.com/llvm/llvm-project/pull/114357