[llvm] [memcpyopt] handle memcpy from memset in more cases (PR #140954)

[Jameson Nash via llvm-commits]

vtjnash wrote:

Second part (sizes aren't constant) is stacked on top of this PR, for once this is merged:
https://github.com/vtjnash/llvm-project/compare/jn/memcpyopt-memset-offset...vtjnash:llvm-project:jn/memcpyopt-memset-nonconst-offset?expand=1

Third part (MOffset < 0) seems potentially too annoying to be worth doing, unless you want it. But maybe it is worth just doing the memcpy->memset replacement, without also trying to computed the optimized the start and end points?

https://github.com/llvm/llvm-project/pull/140954