Properly specify the regex used to match register indexes.

Summary: Something like "core:1" would match and try to be interpreted by the following code otherwise.

Test Plan: Run tests and make sure the ones failing previously now pass.

Reviewers: tfiala, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D6257

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@221980 91177308-0d34-0410-b5e6-96231b3b80d8