[llvm] Instruction: avoid crash in getStableDebugLoc when `this` isn't a DbgInfoIntrinsic (PR #66266)

[Fangrui Song via llvm-commits]

MaskRay wrote:

The new code seems to preserve the behavior prior to https://reviews.llvm.org/D159485 so seems safe. We should need proper tests for this case, perhaps @jmorse (https://reviews.llvm.org/D159485) can help on it. But I can give mine if Rust urgently needs this.

https://github.com/llvm/llvm-project/pull/66266