[mips][microMIPS] Fix bugs related to atomic SC/LL instructions

Fix bugs related to atomic microMIPS SC/LL instructions: While expanding atomic
operations the mips32r2 encoding was emitted instead of microMIPS.

Differential Revision: http://reviews.llvm.org/D6659


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224524 91177308-0d34-0410-b5e6-96231b3b80d8