[mips][msa] Added MSA.txt to describe instruction selection quirks.

This file contains notes about the instruction selection for MSA. For example,
it notes that ilvl.d is cannot be selected because ilvev.d covers the same
cases and is selected instead of ilvl.d.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191507 91177308-0d34-0410-b5e6-96231b3b80d8