Hexagon: Move HexagonMCInst.h to MCTargetDesc/HexagonMCInst.h.
Add HexagonMCInst class which adds various Hexagon VLIW annotations.
In addition, this class also includes some APIs related to the
constant extenders.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175634 91177308-0d34-0410-b5e6-96231b3b80d8