Avoid accessing LLVM/DWARF register mappings if undefined
If the backend does not define LLVM/DWARF register mappings, the associated
variables are undefined since the map initializer is called by auto-generated
TableGen routines. This patch initializes the pointers and sizes to nullptr
and zero, respectively, and checks that they are valid before searching
for a mapping.
Reviewers: grosbach, dschuff
Differential Revision: https://reviews.llvm.org/D23458