[Mips] Remove uneeded variants of ADDC/ADDE lowering

Summary: As it turns out, the lowering for the Mips16* family of target is the exact same thing as what the ops expands to, so the code handling them can be removed and the ops only enabled for the MipsSE* family of targets.

Reviewers: smaksimovic, atanasyan, abeserminji

Subscribers: sdardis, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D47703

llvm-svn: 334052