One more patch towards JIT support for Mips.

One more patch towards JIT support for Mips.
- Add TSFlags for the instruction formats. The idea here is to use
  as much encoding as possible from getBinaryCodeForInstr, and having
  TSFLags formats for that would make it easier to encode most part
  of the instructions (since Mips encodings are pretty straightforward)
- Improve the mips mechanism for compilation callback
- Add Mips specific code for invalidating the instruction cache
- Next patch will address wrong tablegen encoding

Commit msg added by my own but the patch is from Sasa Stankovic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@139688 91177308-0d34-0410-b5e6-96231b3b80d8