Quiet Coverity

Summary: Coverity warns that unsigned >= 0 is always true, and k_first_gpr_powerpc happens to be 0.  Quiet Coverity by changing that comparison instead to a static_assert(), in case things change in the future.

Reviewers: emaste

Reviewed By: emaste

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D7576

llvm-svn: 228908