[mips][msa] Fix immediate value of LSA instruction as it was being wrongly encoded.
    
The immediate field should be encoded as "imm - 1" as the CPU always adds one to that field.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195004 91177308-0d34-0410-b5e6-96231b3b80d8