[llvm] [RISCV][VLOPT] Don't reduce the VL is the same as CommonVL (PR #123878)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

I appreciate all of the comments here. I agree that we should improve how we manage our worklist. This was something that @preames pointed out as an area for improvement in the original addition of this patch. I'd like to land this patch independently of any improvements to the management of the worklist. I think that this change and a worklist change are beneficial on their own.

Thank you @lukel97 for pointing out that this is not a case of infinite loop, but a case that is pathologically slow. I've added the pathologically slow test case to this PR, since this PR fixes the slowness problem.

https://github.com/llvm/llvm-project/pull/123878