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

[Artem Belevich via llvm-commits]

================
@@ -70,14 +70,30 @@ bool NVPTXLowerAlloca::runOnFunction(Function &F) {
     for (auto &I : BB) {
       if (auto allocaInst = dyn_cast<AllocaInst>(&I)) {
         Changed = true;
+
----------------
Artem-B wrote:

I would add an overall comment here describing what we do here and why. Otherwise it's not obvious why we do the seemingly pointless casting to AS5 and back.

Basically, we need to make sure that LLVM has info that alloca is in AS(5), but alloca's users still need a generic pointer to operate on.

For allocas in generic AS, we add asc to AS(5) and back to generic.
For allocas in AS5, we just need an ASC to generic.

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