[Hexagon] Adding zxth instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222662 91177308-0d34-0410-b5e6-96231b3b80d8
[Hexagon] Adding aslh instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222668 91177308-0d34-0410-b5e6-96231b3b80d8