Make sure that DSUB does not duplicate the pattern of DSUBU

Test Plan:
Run test suite to make sure there is no regression.
https://dmz-portal.mips.com/bb/builders/LLVM%20with%2064bit%20and%20delay%20slot%20optimizer%20and%20direct%20object%20emitter/builds/626

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D3497

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207247 91177308-0d34-0410-b5e6-96231b3b80d8