unsolved compiler performance problem: Const-propagation + branch-unrolling

[ghost]

I'm not sure how to title this issue, but i just wanted to open an issue to track the unresolved problem we discovered today, @JeffBezanson and @vtjnash.

Namely, when a large (100 fields?) struct is const-propagated, and also involved in a branch, the LLVM IR seems to explode with lots and lots of phi nodes. (We saw like 10,000 in our function.)

In our case, the struct Const was only present because of issue #31418. However, it is valid to have such a large Const struct, and that might trigger this case in the future.

I'm opening this issue to track the problem so that we don't lose context on it. :)