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

[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.

llvm-svn: 204792