[mips] Move IsFP64bit/NotFP64bit to the front of the AdditionalPredicates list

Summary:
This makes it easier to prove a more complicated change in the next commit
is non-functional.

Reviewers: vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3639

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208197 91177308-0d34-0410-b5e6-96231b3b80d8