[mips] Add synci instruction.

Patch by Amaury Pouly

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6421


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222899 91177308-0d34-0410-b5e6-96231b3b80d8