[mips] Fold FeatureSEInReg into FeatureMips32r2

Summary: No functional change

Reviewers: vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3693

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208543 91177308-0d34-0410-b5e6-96231b3b80d8