This is a follow-up to r203635. Saleem pointed out that since symbolic register
names are much easier to read, it would be good if we could turn them off only
when we really need to because we're using an external assembler.
Differential Revision: http://llvm-reviews.chandlerc.com/D3056
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203806 91177308-0d34-0410-b5e6-96231b3b80d8