Revert r349041: [tblgen][disasm] Separate encodings from instructions

One of the GCC based bots is objecting to a vector of const EncodingAndInst's:
In file included from /usr/include/c++/8/vector:64,
                 from /export/users/atombot/llvm/clang-atom-d525-fedora-rel/llvm/utils/TableGen/CodeGenInstruction.h:22,
                 from /export/users/atombot/llvm/clang-atom-d525-fedora-rel/llvm/utils/TableGen/FixedLenDecoderEmitter.cpp:15:
/usr/include/c++/8/bits/stl_vector.h: In instantiation of 'class std::vector<const {anonymous}::EncodingAndInst, std::allocator<const {anonymous}::EncodingAndInst> >':
/export/users/atombot/llvm/clang-atom-d525-fedora-rel/llvm/utils/TableGen/FixedLenDecoderEmitter.cpp:375:32:   required from here
/usr/include/c++/8/bits/stl_vector.h:351:21: error: static assertion failed: std::vector must have a non-const, non-volatile value_type
       static_assert(is_same<typename remove_cv<_Tp>::type, _Tp>::value,
                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/8/bits/stl_vector.h:354:21: error: static assertion failed: std::vector must have the same value_type as its allocator
       static_assert(is_same<typename _Alloc::value_type, _Tp>::value,
                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

llvm-svn: 349046