[mips] Move the CHECK lines in mips*-register-names.s to make it more obvious which CHECK matches with which insn

This reveals a small mistake in mips-register-names.s ($sp is tested twice and
$s8 is not tested) which will be fixed in a follow-up commit.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204792 91177308-0d34-0410-b5e6-96231b3b80d8