[mips] Implementation of dli.
    
Patch by David Chisnall
His work was sponsored by: DARPA, AFRL
    
Some small modifications to the original patch: we now error if
it's not possible to expand an instruction (mips-expansions-bad.s has some
examples). Added some comments to the expansions.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211271 91177308-0d34-0410-b5e6-96231b3b80d8