Fixes to r153903.  Added missing explanation of behaviour when the VirtRegMap is NULL.  Also changed it in this case to just avoid updating the map, but live ranges or intervals will still get updated and created

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153914 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed