[codeview] don't try to emit variable locations without registers

This fixes a problem introduced 311957, where the compiler would crash
with "fatal error: error in backend: unknown codeview register".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311969 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed