The current constraint on the `init_ordered' function accepts only
numeric types. While one still can init a non-numerically-keyed tree by
passing a comparator explicitly, the reason to disallow non-numeric
ordered types is nowhere to be found.
There was a disconnect between the declared return type for alloca
intrinsic in check_builtin.cpp (multi_pointer(t_u8)) and the generated
result type in llvm_backend_proc.cpp (t_u8_ptr).
This allowed slicing the return type, but in the code generation process
the type of the expression wasn't sliceable, which triggered the assert.
Fixes#2139