[AMDGPU][LowerBufferFatPointers] Fix crash with select false (#166471)

If the input to LowerBufferFatPointers is such that the resource- and
offset-specific `select` instructions generated for a `select` on `ptr
addrspae(7)` fold away, the pass would crash when trying to replace an
instruction with itself. This commit resolves the issue.

Fixes iree-org/iree#22551

Author: krzysz00

llvm/lib/Target/AMDGPU/AMDGPULowerBufferFatPointers.cpp

@@ -1565,8 +1565,11 @@ void SplitPtrStructs::processConditionals() {
     } else if (isa<SelectInst>(I)) {
       if (MaybeRsrc) {
         if (auto *RsrcInst = dyn_cast<Instruction>(Rsrc)) {
-          ConditionalTemps.push_back(RsrcInst);
-          RsrcInst->replaceAllUsesWith(*MaybeRsrc);
+          // Guard against conditionals that were already folded away.
+          if (RsrcInst != *MaybeRsrc) {
+            ConditionalTemps.push_back(RsrcInst);
+            RsrcInst->replaceAllUsesWith(*MaybeRsrc);
+          }
         }
         for (Value *V : Seen)
           FoundRsrcs[V] = *MaybeRsrc;

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

@@ -481,3 +481,15 @@ define void @dominance_not_in_program_order(ptr addrspace(7) inreg %arg) {
   %lsr.iv11 = phi ptr addrspace(7) [ %arg, %.loopexit ], [ %arg, %.preheader15 ]
   br label %.loopexit
 }
+
+;; iree-org/iree#22551 - crash on something that reduces to the below non-canonical select.
+define ptr addrspace(7) @noncanonical_const_cond(ptr addrspace(7) %x) {
+; CHECK-LABEL: define { ptr addrspace(8), i32 } @noncanonical_const_cond
+; CHECK-SAME: ({ ptr addrspace(8), i32 } [[RET:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT:    [[X_RSRC:%.*]] = extractvalue { ptr addrspace(8), i32 } [[RET]], 0
+; CHECK-NEXT:    [[X_OFF:%.*]] = extractvalue { ptr addrspace(8), i32 } [[RET]], 1
+; CHECK-NEXT:    ret { ptr addrspace(8), i32 } [[RET]]
+;
+  %ret = select i1 false, ptr addrspace(7) %x, ptr addrspace(7) %x
+  ret ptr addrspace(7) %ret
+}