[Hexagon] Adding post-increment signed byte loads with tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224866 91177308-0d34-0410-b5e6-96231b3b80d8
[Hexagon] Adding post-increment unsigned byte loads.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224867 91177308-0d34-0410-b5e6-96231b3b80d8