[llvm] [DSE] Stop double-counting the dead slice offset in shortenAssignment (PR #216227)

[Eric Christopher via llvm-commits]

echristo wrote:

> I feel I'm being a bit pedantic with my nits, sorry!

Not a single problem at all. Good comments are important.

> 
> Just out of curiosity, because you mention the patch is assisted by AI, does that extend to the comments in the tests, or are these all human-written?

A little bit of both honestly. I've always felt like I've under-commented my testcases (10 years later - "what on earth did that mean?") and so perhaps went a little overboard with some help. Thanks for checking in and for the comment review. It's appreciated. I've gone ahead and updated that first section - see if that's a bit better explanation of the what the test case is designed to test :)

https://github.com/llvm/llvm-project/pull/216227