[llvm] [LoopUnroll] Clamp PartialThreshold for large LoopMicroOpBufferSize (PR #67657)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

I've always struggled with the idea of LoopMicroOpBufferSize in general. It appears to be just used as a way of throttling unrolls, to avoid the potential cost of unnecessary compare+branch. In which case shouldn't we be trying to better estimate the cost of the loop control flow, based off branch distance etc? That way we avoid estimation issues for cpus without a dedicated loopback buffer etc.

https://github.com/llvm/llvm-project/pull/67657