[mips] Tidy up cnMIPS tablegen definitions. NFC.

Summary:
In particular, make the cnMIPS predicates much more obvious and prefer
  def ... : ... {
    let Foo = bar;
  }
over:
  let Foo = bar in
  def ... : ...;

Reviewers: vkalintiris

Subscribers: dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D18354

llvm-svn: 264258