Add function that translates subregister lane masks to other subregs.

This works like the composeSubRegisterIndices() function but transforms
a subregister lane mask instead of a subregister index.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223874 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed