[AMDGPU] Prevent re-visits in LowerBufferFatPointers

Fixes iree-org/iree#22001

The visitor in SplitPtrStructs would re-visit instructions if an
instruction earlier in program order caused a recursive visit() call
via getPtrParts(). This would cause instructions to be processed
multiple times.

As a consequence of this, PHI nodes could be added to the Conditionals
array multiple times, which would to a conditinoal that was already
simplified being processed multiple times. After the code moved to
InstSimplifyFolder, this re-processing, combined with more agressive
simplifications, would lead to an attempt to replace an instruction
with itself, causing an assertion failure and crash.

This commit resolves the issue and adds the reduced form of the
crashing input as a test.

Author: krzysz00

llvm/lib/Target/AMDGPU/AMDGPULowerBufferFatPointers.cpp

@@ -2328,6 +2328,12 @@ void SplitPtrStructs::processFunction(Function &F) {
   LLVM_DEBUG(dbgs() << "Splitting pointer structs in function: " << F.getName()
                     << "\n");
   for (Instruction *I : Originals) {
+    // In some cases, instruction order doesn't reflect program order,
+    // so the visit() call will have already visited coertain instructions
+    // by the time this loop gets to them. Avoid re-visiting these so as to,
+    // for example, avoid processing the same conditional twice.
+    if (SplitUsers.contains(I))
+      continue;
     auto [Rsrc, Off] = visit(I);
     assert(((Rsrc && Off) || (!Rsrc && !Off)) &&
            "Can't have a resource but no offset");

llvm/test/CodeGen/AMDGPU/lower-buffer-fat-pointers-control-flow.ll

@@ -455,3 +455,29 @@ loop:
 exit:
   ret float %sum
 }
+
+define void @dominance_not_in_program_order(ptr addrspace(7) inreg %arg) {
+; CHECK-LABEL: define void @dominance_not_in_program_order
+; CHECK-SAME: ({ ptr addrspace(8), i32 } inreg [[ARG:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT:  .preheader15:
+; CHECK-NEXT:    [[ARG_RSRC:%.*]] = extractvalue { ptr addrspace(8), i32 } [[ARG]], 0
+; CHECK-NEXT:    [[ARG_OFF:%.*]] = extractvalue { ptr addrspace(8), i32 } [[ARG]], 1
+; CHECK-NEXT:    br label [[DOTLR_PH18:%.*]]
+; CHECK:       .loopexit:
+; CHECK-NEXT:    [[SCEVGEP12:%.*]] = add i32 [[LSR_IV11_OFF:%.*]], 16
+; CHECK-NEXT:    br label [[DOTLR_PH18]]
+; CHECK:       .lr.ph18:
+; CHECK-NEXT:    [[LSR_IV11_OFF]] = phi i32 [ [[ARG_OFF]], [[DOTLOOPEXIT:%.*]] ], [ [[ARG_OFF]], [[DOTPREHEADER15:%.*]] ]
+; CHECK-NEXT:    br label [[DOTLOOPEXIT]]
+;
+.preheader15:
+  br label %.lr.ph18
+
+.loopexit:                                        ; preds = %.lr.ph18
+  %scevgep12 = getelementptr i8, ptr addrspace(7) %lsr.iv11, i32 16
+  br label %.lr.ph18
+
+.lr.ph18:                                         ; preds = %.loopexit, %.preheader15
+  %lsr.iv11 = phi ptr addrspace(7) [ %arg, %.loopexit ], [ %arg, %.preheader15 ]
+  br label %.loopexit
+}