[mips] Fix ll and sc instructions

Summary: The ll and sc instructions for r6 and non-r6 are misplaced. This patch fixes that.

Patch by Jyun-Yan You

Differential Revision: http://reviews.llvm.org/D4578


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213847 91177308-0d34-0410-b5e6-96231b3b80d8