[x86] Hoist the logic for extracting the relevant bits of information
from the MachineInstr into the caller which is already doing a switch
over the instruction.

This will make it more clear how to compute different operands to feed
the comment selection for example.

Also, in a drive-by-fix, don't append an empty comment string (which is
a no-op ultimately).

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218361 91177308-0d34-0410-b5e6-96231b3b80d8