[mips] Fold FeatureBitCount into FeatureMips32 and FeatureMips64

Summary:
DCL[ZO] are now correctly marked as being MIPS64 instructions. This has no
effect on the CodeGen tests since expansion of i64 prevented their use
anyway.

The check for MIPS16 to prevent the use of CLZ no longer prevents DCLZ as
well. This is not a functional change since DCLZ is still prohibited by
being a MIPS64 instruction (MIPS16 is only compatible with MIPS32).

No functional change

Reviewers: vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3694