[compiler-rt] [llvm] [MSAN] support vpermilvar AVX instructions (PR #143053)

[Thurston Dang via llvm-commits]

https://github.com/thurstond requested changes to this pull request.

If the permutation is uninitialized, the DoL should reject the PERM. There's two schools of thought:
- use `insertShadowCheck(I.getArgOperand(1))`, which will report it as uninitialized memory; similar to `extractLowerShadow`, where "intuitively it feels like a partly uninitialized mask is unintended, and we should warn the user immediately."
- mix in the shadow of the permutation and propagate the shadow (don't crash); similar to `handleIntrinsicByApplyingToShadow`. In the case of an uninitialized permutation, arguably all bits should become uninitialized.

https://github.com/llvm/llvm-project/pull/143053