[mips] wrong opcode for ll/sc instructions on mipsr6 when -integrated-as is used

Summary:
This commit resolves wrong opcodes for ll and sc instructions for r6 architecutres, which were generated in method MipsTargetLowering::emitAtomicBinary.

Author: Jelena.Losic

Reviewers: dsanders

Subscribers: dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D13593

llvm-svn: 251629