astutils.cpp: optimized visitAstNodesGeneric() a bit

[firewave]

I tested this with cli/filelister.cpp. I wanted to test it with more files but with all the recent performance regressions it's agonizing slow while running in valgrind.

The initial Ir count was 353,306,619.

Reducing the std::stack operations by not adding nullptr entries reduced it to 334,276,673 even though the checks added some overhead.

Switching the backend of the std::stack from std::deque to std::vector reduced it to 293,371,232. The main problem is that the construction of a std::deque is much more expensive. The destruction cost stays the same.

So in total this saves us about 12% of the Ir count.

The main issue here still remains the creation and destruction of std::stack which still amounts to more than 22% of the total Ir count. So it's the same issue we are seeing with short-living small std::vector as related to in #3432. It is also still the single most expensive function we call.

[firewave]

These numbers apply to Clang 13 - with GCC 11 this speed-up in switching the container does not happen. I will try to dig into it and report it upstream.

I have some further optimization which improves the speed for GCC to match current Clang as well as improving Clang.

[firewave]

While looking into this I came across more code generation differences between the compilers like llvm/llvm-project#53268.