[decompiler] clean up vector detection and add vector float product detection (#638)

* change

* recognize vector float product and update tests

decompiler/analysis/insert_lets.cpp

@@ -336,6 +336,23 @@ FormElement* fix_up_abs_2(LetElement* in, const Env& env, FormPool& pool) {
   return in;
 }
 
+FormElement* rewrite_empty_let(LetElement* in, const Env&, FormPool&) {
+  if (in->entries().size() != 1) {
+    return nullptr;
+  }
+
+  if (!in->body()->elts().empty()) {
+    return nullptr;
+  }
+
+  auto reg = in->entries().at(0).dest.reg();
+  if (reg.get_kind() == Reg::GPR && !reg.allowed_local_gpr()) {
+    return nullptr;
+  }
+
+  return in->entries().at(0).src->try_as_single_element();
+}
+
 /*!
  * Attempt to rewrite a let as another form.  If it cannot be rewritten, this will return nullptr.
  */
@@ -360,6 +377,11 @@ FormElement* rewrite_let(LetElement* in, const Env& env, FormPool& pool) {
     return as_abs_2;
   }
 
+  auto as_unused = rewrite_empty_let(in, env, pool);
+  if (as_unused) {
+    return as_unused;
+  }
+
   // nothing matched.
   return nullptr;
 }