Fix the root cause of the bootstrap failure:

Fix the root cause of the bootstrap failure:

There was no way to check if a given register/mode pair was valid. We now return
an error code (-2) instead of asserting. If anyone thinks that an assert
at this point  is really needed, we can autogen a hasValidDwarfRegNum instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132236 91177308-0d34-0410-b5e6-96231b3b80d8