[TableGen] Generate offsets into a flat array for getOperandType

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@366278 91177308-0d34-0410-b5e6-96231b3b80d8