[Hexagon] Adding compare byte/halfword reg-reg/reg-imm forms.  Adding compare to general register reg-imm form.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224991 91177308-0d34-0410-b5e6-96231b3b80d8
[Hexagon] Dropping old combine instructions without encodings.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224992 91177308-0d34-0410-b5e6-96231b3b80d8