[Custom Descriptors] Propagate subtyping to describees in Unsubtyping

[tlively]

An upcoming change to the custom descriptor validation rules will
require describees to be in a subtype relationship whenever their
descriptors are. This is similar to the requirement we already have in
the opposite direction. Update Unsubtyping to respect this upcoming
rule.

[kripken]

Fuzz bug bisected to here:

(module
 (rec                        
  (type $0 (sub (descriptor $1 (struct (field (mut (ref null $1))) (field (mut i16)) (field (mut (ref null $0))) (field i8) (field (mut i31ref)) (field (mut i64))))))
  (type $1 (sub (describes $0 (descriptor $3 (struct)))))
  (type $2 (sub (struct (field (ref null $4)) (field externref))))
  (type $3 (sub (describes $1 (descriptor $4 (struct (field i32))))))
  (type $4 (sub (describes $3 (struct (field (mut i64)) (field (mut i8))))))
  (type $5 (sub (func (result (ref $0)))))
  (type $6 (func (param i64 f32 f32 (ref null $4) (ref null $0) (ref $4)) (result v128)))
  (type $7 (array (mut (ref null $7))))
 )
 (rec
  (type $8 (struct (field (mut i8)) (field (mut (ref $7))) (field (mut i64))))
  (type $9 (struct (field v128)))
  (type $10 (descriptor $12 (struct (field (mut i8)) (field (mut v128)) (field (ref array)) (field i8))))
  (type $11 (array i16))
  (type $12 (sub (describes $10 (descriptor $13 (struct (field (ref $19)) (field arrayref) (field (ref null $6)) (field i64) (field (ref null $13)) (field (mut i8)))))))
  (type $13 (sub (describes $12 (descriptor $17 (struct (field i8) (field i8) (field (mut (ref null $12))))))))
  (type $14 (func (param f32 (ref null $10)) (result v128)))
  (type $15 (array (mut externref)))
  (type $16 (array externref))
  (type $17 (sub (describes $13 (struct (field i8) (field (mut f32)) (field (ref $7))))))
  (type $18 (sub (array i8)))
  (type $19 (sub (struct (field (mut eqref)) (field (mut i16)) (field (ref $21)))))
  (type $20 (sub (func (param (ref $22) (ref null $8) f32 f64 i32) (result (ref func)))))
  (type $21 (sub (array i16)))
  (type $22 (sub (func (param (ref null $4)) (result (ref null $21)))))
 )
 (rec
  (type $23 (sub $18 (array i8)))
  (type $24 (func (result (ref $4))))
  (type $25 (sub final $0 (descriptor $26 (struct (field (mut (ref null $1))) (field (mut i16)) (field (mut (ref null $0))) (field i8) (field (mut i31ref)) (field (mut i64))))))
  (type $26 (sub final $1 (describes $25 (descriptor $29 (struct (field f64) (field i32))))))
  (type $27 (sub final $20 (func (param (ref $22) eqref f32 f64 i32) (result (ref nofunc)))))
  (type $28 (array (ref null $31)))
  (type $29 (sub final $3 (describes $26 (descriptor $30 (struct (field i32) (field f32) (field (ref $3)) (field (mut i32)) (field i31ref))))))
  (type $30 (sub final $4 (describes $29 (descriptor $31 (struct (field (mut i64)) (field (mut i8)) (field (mut externref)) (field (mut (ref func))) (field i16))))))
  (type $31 (describes $30 (descriptor $32 (struct))))
  (type $32 (sub $3 (describes $31 (descriptor $33 (struct (field i32) (field (ref $33)) (field (mut i16)) (field (mut i64)))))))
  (type $33 (sub final $4 (describes $32 (struct (field (mut i64)) (field (mut i8)) (field i64) (field (mut (ref $18)))))))
 )
 (type $34 (func (result (ref null $25))))
 (global $global$0 (ref (exact $0)) (struct.new_default $0
  (struct.new_default $1
   (ref.null none)
  )
 ))
 (export "func_67" (func $0))
 (func $0 (type $34) (result (ref null $25))
  (unreachable)
 )
)

$ bin/wasm-opt a.wat -all --unsubtyping
wasm-opt: src/passes/Unsubtyping.cpp:516: void wasm::{anonymous}::Unsubtyping::process(wasm::HeapType, wasm::HeapType): Assertion `HeapType::isSubType(sub, super)' failed.
Aborted (core dumped)

@tlively

[tlively]

Fixed by #7932.