[Hexagon] Adding functionality for duplexing. Duplexing is a way to compress...

[Hexagon] Adding functionality for duplexing.  Duplexing is a way to compress commonly used pairs of instructions in order to reduce code size.  The test case duplex.ll normally would be 8 bytes, assign register to 0 and jump to link register.  After duplexing this is only 4 bytes.  This also tests the HexagonMCShuffler code path which is used to make sure duplexed instructions still follow slot requirements.

llvm-svn: 239095