R600/SI: Add check for low 32 bits of encoding to mubuf tests

R600/SI: Add check for low 32 bits of encoding to mubuf tests

There are no variable values like registers encoded in the low 32 bits of MUBUF
instructions, so it is relatively easy to check these bits, and it will
help prevent us from introducing encoding bugs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215397 91177308-0d34-0410-b5e6-96231b3b80d8