[llvm] [NVPTX] Check Before inserting AddrSpaceCastInst in NVPTXLoweringAlloca (PR #106127)

[weiwei chen via llvm-commits]

================
@@ -72,12 +72,22 @@ bool NVPTXLowerAlloca::runOnFunction(Function &F) {
         Changed = true;
         auto ETy = allocaInst->getAllocatedType();
         auto LocalAddrTy = PointerType::get(ETy, ADDRESS_SPACE_LOCAL);
-        auto NewASCToLocal = new AddrSpaceCastInst(allocaInst, LocalAddrTy, "");
-        auto GenericAddrTy = PointerType::get(ETy, ADDRESS_SPACE_GENERIC);
-        auto NewASCToGeneric =
-            new AddrSpaceCastInst(NewASCToLocal, GenericAddrTy, "");
-        NewASCToLocal->insertAfter(allocaInst);
-        NewASCToGeneric->insertAfter(NewASCToLocal);
+        PointerType *AllocInstPtrTy = dyn_cast_or_null<PointerType>(
----------------
weiweichen wrote:

It comes from [here](https://github.com/llvm/llvm-project/blob/7854b16d2699ca7cc02d4ea066230d370c751ba9/llvm/lib/IR/Instructions.cpp#L3416)

```
AddrSpaceCastInst::AddrSpaceCastInst(Value *S, Type *Ty, const Twine &Name,
                                     InsertPosition InsertBefore)
    : CastInst(Ty, AddrSpaceCast, S, Name, InsertBefore) {
  assert(castIsValid(getOpcode(), S, Ty) && "Illegal AddrSpaceCast");
}
```

where `castIsValid(getOpcode(), S, Ty)` returns `false` if `S` is already in the same address space which is from the logic [here](https://github.com/llvm/llvm-project/blob/7854b16d2699ca7cc02d4ea066230d370c751ba9/llvm/lib/IR/Instructions.cpp#L3333-L3334)

```
    if (SrcPtrTy->getAddressSpace() == DstPtrTy->getAddressSpace())
      return false;
```
@justinfargnoli, would it be better to remove ☝️ instead?


https://github.com/llvm/llvm-project/pull/106127