[AMDGPU][LowerBufferFatPointers] Fix lack of rewrite when loading/storing null

Fixes #154056.

The fat buffer lowering pass was erroniously detecting that it did not
need to run on functions that only load/store to the null constant (or
other such constants). We thought this would be covered by
specilaizing constants out to instructions, but that doesn't account
foc trivial constants like null. Therefoe, we check the operands of
instructions for buffer fat pointers in order to find such constants
and ensure the pass runs.

Author: krzysz00

llvm/lib/Target/AMDGPU/AMDGPULowerBufferFatPointers.cpp

@@ -2366,8 +2366,12 @@ static bool containsBufferFatPointers(const Function &F,
                                       BufferFatPtrToStructTypeMap *TypeMap) {
   bool HasFatPointers = false;
   for (const BasicBlock &BB : F)
-    for (const Instruction &I : BB)
+    for (const Instruction &I : BB) {
       HasFatPointers |= (I.getType() != TypeMap->remapType(I.getType()));
+      // Catch null pointer constasts in loads, stores, etc.
+      for (const Value *V : I.operand_values())
+        HasFatPointers |= (V->getType() != TypeMap->remapType(V->getType()));
+    }
   return HasFatPointers;
 }