fix #32076, compile times of long static vectors

[JeffBezanson]

This adds some heuristics to do less constant propagation. I did some profiling to see which functions get constant prop'd the most, then tried to exclude cases that don't seem useful:

• getindex and setindex! of non-const arrays (the getindex part fixes Compilation times of long static vectors are increasing #32076)
• iterate of a non-constant iterator
• arithmetic and comparisons with some non-const arguments. It seems to me you need all arguments to be constant to get anything in these cases.

[stev47]

This bit me. Is there a way to opt into constant propagation for getindex?
As soon as I'm subtyping AbstractArray, getindex loses constant propagation. I could change the user-facing indexing syntax but that seems awkward to do just for efficiency reasons.
I tried to work around by defining Base.issingletontype(...) = true (technically wrong since my type is non-singleton) but that does not seem to have any effect.