[llvm] [LV][EVL] Reimplement method for extracting new mask. nfc (PR #156827)

[Luke Lau via llvm-commits]

lukel97 wrote:

Btw, is this PR NFC? I think this is good to have but it looks to be more powerful than the existing logic. Does it catch remove any more header masks in llvm-test-suite/SPEC?

https://github.com/llvm/llvm-project/pull/156827