[llvm] [NFC] Add GIntrinsic::setIntrinsicID (PR #106236)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

It depends on your use-case. In the combiner, we have several ways to mutate or replace instructions. In your file, I believe `dropPoisonGeneratingFlags` is the only method  that modifies instructions. The others are getters and testers.

https://github.com/llvm/llvm-project/pull/106236