[mips] Fold FeatureSwap into FeatureMips32r2 and FeatureMips64r2

Summary:
dsbh and dshd are not available on Mips32r2. No codegen test changes
required since expansion of i64 prevented the use of these instructions
anyway.

Depends on D3690

Reviewers: vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3692