[llvm] [SDAG] Avoid crash when creating debug fragements for scalable vectors (PR #165233)

[Amara Emerson via llvm-commits]

aemerson wrote:

That may be the case for clang, but not every user of SME is going to be adhering to the AAPCS. That's a platform level decision and not a restriction on the IR semantics. LLVM shouldn't crash on any legal IR input, and otherwise the IR verifier should reject it as malformed.

https://github.com/llvm/llvm-project/pull/165233