[Hexagon] Replacing the remaining postincrement stores with versions that have encoding bits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224951 91177308-0d34-0410-b5e6-96231b3b80d8
[Hexagon] Adding post-increment register form stores and register-immediate form stores with tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224952 91177308-0d34-0410-b5e6-96231b3b80d8
Remove unnecessary StringRef->std::string conversion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224953 91177308-0d34-0410-b5e6-96231b3b80d8