[llvm] [LV][EVL] Introduce the EVLIndVarSimplify Pass for EVL-vectorized loops (PR #131005)

[Luke Lau via llvm-commits]

lukel97 wrote:

I think these should be the SPEC 2017/TSVC changes from this patch: https://lnt.lukelau.me/db_default/v4/nts/568

There's a lot of code size improvements and a 2% improvement in 525.x264_r. 

However there's also a regression in 523.xalancbmk_r and [one of the TSVC tests](https://lnt.lukelau.me/db_default/v4/nts/568/graph?test.475=3). 

No need to revert or anything, I'm just posting this here so we don't forget to take a look at these. Thanks for pushing this through!

https://github.com/llvm/llvm-project/pull/131005