[llvm] [InstCombine][CVP] Fold length-one memset with variable fill (PR #213240)

[Antonio Frighetto via llvm-commits]

antoniofrighetto wrote:

Still needs a rebase to main, after the other patch landed. Please also update the PR title and description to reflect the current code (the testing bullet list can go). Please extract the transform into a dedicated helper (e.g. foldMemSetZeroOrOneLength), alongside the other transforms in foldUnusualPatterns work (rather than collecting candidates and processing them separately, as already mentioned).

https://github.com/llvm/llvm-project/pull/213240