fall back to dynamic dispatch in union-splitting optimization

This should help #17932. If a function returns an unanticipated type
that wasn't part of the `Union` seen by inference, do dynamic
dispatch instead of erroring.

(cherry picked from commit b0cf591)
ref #18046

base/inference.jl

@@ -9,7 +9,8 @@ const MAX_TUPLETYPE_LEN = 15
 const MAX_TUPLE_DEPTH = 4
 
 const MAX_TUPLE_SPLAT = 16
-const MAX_UNION_SPLITTING = 6
+const MAX_UNION_SPLITTING = 4
+const UNION_SPLIT_MISMATCH_ERROR = false
 
 # alloc_elim_pass! relies on `Slot_AssignedOnce | Slot_UsedUndef` being
 # SSA. This should be true now but can break if we start to track conditional
@@ -2437,7 +2438,7 @@ function inlineable(f::ANY, ft::ANY, e::Expr, atypes::Vector{Any}, sv::Inference
                                 all = false
                             end
                         end
-                        if all
+                        if UNION_SPLIT_MISMATCH_ERROR && all
                             error_label === nothing && (error_label = genlabel(sv))
                             push!(stmts, GotoNode(error_label.label))
                         else