[TableGen] Generate offsets into a flat array for getOperandType
Rather than an array of std::initializer_list, generate a table of
offsets and a flat array of the operands for getOperandType. This is a
bit more efficient on platforms that don't manage to get the array of
inintializer_lists initialized at link time (I'm looking at you
macOS). It's also quite quite a bit faster to compile.