Before this commit sirit generated a stream of tokens that would then be
inserted to the final SPIR-V binary. This design was carried from the
initial design of manually inserting opcodes into the code. Now that
all instructions but labels are inserted when their respective function
is called, the old design can be dropped in favor of generating a valid
stream of SPIR-V opcodes.
The API for variables is broken, but adopting the new one is trivial.
Instead of calling OpVariable and then adding a global or local
variable, OpVariable was removed and global or local variables are
generated when they are called.
Avoiding duplicates is now done with an std::unordered_set instead of
using a linear search jumping through vtables.
Previously the test couldn't fail unless it crashed. Now that sirit does
not do work "behind the scenes" that can change between versions (like
declaring capabilities), we can have this checking.
All instructions but OpVariable and OpLabel are automatically emitted.
These functions have to call AddLocalVariable/AddGlobalVariable or
AddLabel respectively.
