[llvm] [AMDGPU] Track buffer resource num_records width in a subtarget field (PR #217136)

[Krzysztof Drewniak via llvm-commits]

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@@ -353,6 +358,14 @@ class GCNSubtarget final : public AMDGPUGenSubtargetInfo,
   bool hasRelaxedBufferOOBMode() const { return BufferOOBRelaxed; }
   bool hasRelaxedTBufferOOBMode() const { return TBufferOOBRelaxed; }
 
+  /// Return the width, in bits, of the num_records field of a buffer resource
+  /// (V#) on this subtarget, or std::nullopt if not yet knows.
+  std::optional<unsigned> getBufferResourceNumRecordsWidth() const {
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krzysz00 wrote:

I like `optional` because it forces people to contend with the generic case, unlike just returning 0 and hoping people remember

https://github.com/llvm/llvm-project/pull/217136