Fix a problem where lib/Target/TargetInstrInfo.h would include and use

a header file from libcodegen.  This violates a layering order: codegen
depends on target, not the other way around.  The fix to this is to 
split TII into two classes, TII and TargetInstrInfoImpl, which defines
stuff that depends on libcodegen.  It is defined in libcodegen, where 
the base is not.

llvm-svn: 45475