[mips][msa] Make LSA_DESC a parameterizable class.

This way it's possible to share the instruction's description for LSA and
DLSA (to be added).

No functional changes.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201078 91177308-0d34-0410-b5e6-96231b3b80d8