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

[Justin Fargnoli via llvm-commits]

justinfargnoli wrote:

Interesting, I would've though that this is allowed. However, the IR Spec says otherwise ([source](https://llvm.org/docs/LangRef.html#id301)):
> The ‘addrspacecast’ instruction takes a pointer or vector of pointer value to cast and a pointer type to cast it to, which must have a different address space.
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> would it be better to remove ☝️ instead?

Since the IR spec does not allow this, we should not remove the check. 

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