[JVM] Don't rely on presence of signatures in JVM IR serialization tests

This patch should not change any reasonable testing behavior. The only
case when the behavior could be changed is when two different functions
have the same IdSignature, which is a problem on its own.

Required for KT-64809

compiler/tests-common-new/tests/org/jetbrains/kotlin/test/backend/handlers/IrInlineBodiesHandler.kt

@@ -22,7 +22,7 @@ import org.jetbrains.kotlin.test.model.TestModule
 import org.jetbrains.kotlin.test.services.TestServices
 
 class IrInlineBodiesHandler(testServices: TestServices) : AbstractIrHandler(testServices) {
-    val declaredInlineFunctionSignatures = mutableSetOf<IdSignature>()
+    val declaredInlineFunctions = hashSetOf<IrSimpleFunction>()
 
     @OptIn(ObsoleteDescriptorBasedAPI::class)
     override fun processModule(module: TestModule, info: IrBackendInput) {
@@ -33,7 +33,7 @@ class IrInlineBodiesHandler(testServices: TestServices) : AbstractIrHandler(test
     }
 
     override fun processAfterAllModules(someAssertionWasFailed: Boolean) {
-        assertions.assertTrue(declaredInlineFunctionSignatures.isNotEmpty())
+        assertions.assertTrue(declaredInlineFunctions.isNotEmpty())
     }
 
     inner class InlineFunctionsCollector : IrElementVisitorVoid {
@@ -42,7 +42,7 @@ class IrInlineBodiesHandler(testServices: TestServices) : AbstractIrHandler(test
         }
 
         override fun visitSimpleFunction(declaration: IrSimpleFunction) {
-            if (declaration.isInline) declaration.symbol.signature?.let { declaredInlineFunctionSignatures.add(it) }
+            if (declaration.isInline) declaredInlineFunctions.add(declaration)
             super.visitSimpleFunction(declaration)
         }
     }
@@ -56,8 +56,8 @@ class IrInlineBodiesHandler(testServices: TestServices) : AbstractIrHandler(test
             val symbol = expression.symbol
             assertions.assertTrue(symbol.isBound)
             val callee = symbol.owner
-            if (callee.symbol.signature in declaredInlineFunctionSignatures) {
-                val trueCallee = (callee as IrSimpleFunction).resolveFakeOverrideOrFail()
+            if (callee is IrSimpleFunction && callee in declaredInlineFunctions) {
+                val trueCallee = callee.resolveFakeOverrideOrFail()
                 assertions.assertTrue(trueCallee.hasBody()) {
                     "IrInlineBodiesHandler: function with body expected"
                 }