structure that represents a mapping without any dependencies on SubRegIndex
numbering.
This brings us closer to being able to remove the explicit SubRegIndex
numbering, and it is now possible to specify any mapping without inventing
*_INVALID register classes.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@104563 91177308-0d34-0410-b5e6-96231b3b80d8