[mips][msa] Fix issue with immediate fields of LD/ST instructions
not being correctly encoded/decoded.
In more detail, immediate fields of LD/ST instructions should be
divided/multiplied by the size of the data format before encoding and
after decoding, respectively.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196494 91177308-0d34-0410-b5e6-96231b3b80d8