This apparently comes up when the register allocator decides that a
variable will become undef along a certain path.
Also improve the error message we emit when we can't map from LLVM
register number to CV register number.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@261016 91177308-0d34-0410-b5e6-96231b3b80d8