Avoid undesired variable shadowing

Michael Sartain refactored RegisterContextPOSIX_* in r192332, and I must
have missed the now-shadowed variable when I rebased the FreeBSD MIPS64
register context after that.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@201334 91177308-0d34-0410-b5e6-96231b3b80d8