[LLDB][MIPS] Merge emulation of similar instructions for MIPS64

    
    SUMMARY:
    This patch merges emulation of similar instructions into a single function (wherever possible) to remove code duplication.
    
    Reviewers: clayborg
    Subscribers: mohit.bhakkad, sagar, jaydeep, lldb-commits
    Differential Revision: http://reviews.llvm.org/D16051

llvm-svn: 257442