Improve typesubtract for tuples (repeat #35600)

[vtjnash]

This re-introduces the improvements #35600—recently reverted—to sharpen the information on Tuple objects through an isa-split (with some improvements of my own towards handling UnionAll over Union), but now limited by the MAX_UNION_SPLITTING parameter to prevent exponential blowup.

[mhauru]

Would there be a downside to making the MAX_UNION_SPLITTING::Int argument of typesubtract optional, with a sensible default, or maybe with one that reproduces the old behavior? I noticed the change because commit 5a56ecd broke a custom version of @inferred in https://github.com/Jutho/TestExtras.jl, that's calling typesubtract. I guess these are pretty deep internals, and the change is very easy to adapt to, but it still seems like unnecessary breakage to me.

[vtjnash]

See af6542e. As you guessed, packages should not rely upon access to Core.Compiler