Change target-specific classes to use more precise static types.

Change target-specific classes to use more precise static types.
This eliminates the need for several awkward casts, including
the last dynamic_cast under lib/Target.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51091 91177308-0d34-0410-b5e6-96231b3b80d8