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