[llvm] Question: What is the correct interpretation of LaneBitmask? (PR #109797)

[Matt Arsenault via llvm-commits]

arsenm wrote:

As it is today, I believe the intent is statement 1. However, I think we would be in a better place if we did require 2. We do have a number of issues whenever there are pieces of registers that aren't fully covered by a defined register, and it is harder to reason about.

We probably need to require fully covering artificial register definitions to fully move the infrastructure to using register units (e.g. replacing livein tracking with regunits instead of registers, and for using regunit masks instead of regmasks) 

https://github.com/llvm/llvm-project/pull/109797