[llvm] [LLVM][AutoUpgrade] Support default args on undeclared multi-call upgrades (PR #216246)

[Rahul Joshi via llvm-commits]

jurahul wrote:

My concern is that this is bundling 2 changes together as a way to test the new code. That is, migration of the collector_b immarg to use default value is a way to test the new changes and not required strictly. OTOH that migration cannot be done before the fix, since it will likely break autoupgrade for these intrinsics. 

Would it make sense instead to add a temporary intrinsic to LLVM that uses a default value and do the AutoUpgrade fix and testing in that context? And keep adoption of MMA intrinsics to use this a separate change?

https://github.com/llvm/llvm-project/pull/216246