[llvm] ValueTracking/test: increase recurrence coverage (PR #108836)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> I didn't try to confirm that the tests are not present elsewhere (a way to do that is to comment out the relevant cases and run tests...)

I did that part of course, and there is some existing but scattered coverage that isn't reduced: I thought it would be good to have a collection of tests testing the recurrence patterns in one place, as a prelude to adding support for other BinOps. Note that although the switch statement has support for FMul, it is impossible to test, as (I think) KnownBits doesn't work with fp values.

https://github.com/llvm/llvm-project/pull/108836