check issimpleenoughtype for PartialStruct during issimplertype

[adienes]

fixes #60715

tmerge_types_slow would check issimpleenoughtype(u) after the PartialStructs were stripped from the types, but that path wasn't getting hit as it would early return via tmerge_fast_path

Claude assisted the analysis but the proposed fix is mine

[vtjnash]

This looks inaccurate of a fix. The tmerge_fast_path code checks issimplertype which should already be a sufficient and superior fix to this. Any idea what input is there? We should add a unit test for that, and not just this integration test

[adienes]

let T(n) = Tuple{Int, Union{Int, Tuple{Int, Union{...}}}} to a nesting depth of 2n (so each of Tuple, Union occurs n times)

then IIUC typea = PartialStruct(T(n), [Const(0), Union{Int, T(n-1)}]) and typeb = PartialStruct(T(n+1), [Const(0), Union{Int, T(n)}])