[fuzzer] ref.func should look up functions by signature

When we generate identical signatures in the fuzzer,
we generate one function for each of the copies.
However, when these functions are added to WasmModulBuilder,
all will be assigned the same signature index.
Therefore, when ref.func tries to find a function corresponding
to a signature index, it will fail, despite a matching signature
existing in the module.
This CL fixes this issue by looking up functions by signature
over signature index.

Bug: v8:11954, chromium:1254387
Change-Id: Iac8d5444d4914d993da63d0630ca4d95e671630c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3197711Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Maria Tîmbur <mtimbur@google.com>
Cr-Commit-Position: refs/heads/main@{#77187}

test/fuzzer/wasm-compile.cc

@@ -850,7 +850,8 @@ class WasmGenerator {
         WasmFunctionBuilder* func = builder_->builder()->GetFunction(i);
         // TODO(11954): Choose a random function from among those matching the
         // signature (consider function subtyping?).
-        if (func->sig_index() == index) {
+        if (*(func->signature()) ==
+            *(builder_->builder()->GetSignature(index))) {
           builder_->EmitWithU32V(kExprRefFunc, func->func_index());
           return;
         }