[mips] Move HasStdEnc to the front of the predicates lists.

Summary:
This will make it easier to prove that a more complicated change in the
following commit is non-functional.

No functional change.

Depends on D3506

Reviewers: vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3549

llvm-svn: 208179