"unknown successor value" assertion in DFA::refinePartitions

[kripken]

Testcase is based off of fuzzing the testcase from https://github.com/WebAssembly/binaryen/issues/7015, reduced.

$ bin/wasm-opt a.wat -all --type-merging --closed-world
wasm-opt: src/support/dfa_minimization.h:75: std::vector<std::vector<T> > wasm::DFA::refinePartitions(const std::vector<std::vector<State<T> > >&) [with T = wasm::HeapType]: Assertion `it != indices.end() && "unknown successor value"' failed.
Aborted

Looks like a DFA minimization issue. @tlively

a.wat.gz