[SystemZ] Tweak instruction format classifications

[SystemZ] Tweak instruction format classifications

There's no real need to have Shift as a separate format type from Binary.
The comments for other format types were too specific and in some cases
no longer accurate.

Just a clean-up, no behavioral change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212707 91177308-0d34-0410-b5e6-96231b3b80d8