Split up complicated assert into parts. (dotnet#50620)

In an effort to narrow down a hard to repro GCStress issue, the assert
is being split into parts.

src/coreclr/vm/stackwalk.cpp

@@ -1496,9 +1496,9 @@ BOOL StackFrameIterator::IsValid(void)
         // In particular, is thread's starting frame the same as it was when
         // we started?
         BOOL bIsRealStartFrameUnchanged =
-            (m_pStartFrame != NULL) ||
-            (m_flags & POPFRAMES) ||
-            (m_pRealStartFrame == m_pThread->GetFrame());
+            (m_pStartFrame != NULL)
+            || (m_flags & POPFRAMES)
+            || (m_pRealStartFrame == m_pThread->GetFrame());
 
 #ifdef FEATURE_HIJACK
         // In GCStress >= 4 two threads could race on triggering GC;
@@ -1508,15 +1508,17 @@ BOOL StackFrameIterator::IsValid(void)
         // In normal case (no GCStress), after p/invoke, IL_STUB will check if GC is in progress and synchronize.
         // NOTE: This condition needs to be evaluated after the previous one to prevent a subtle race condition
         // (https://github.com/dotnet/runtime/issues/11678)
-        bIsRealStartFrameUnchanged = bIsRealStartFrameUnchanged ||
-            ((GCStress<cfg_instr>::IsEnabled()) &&
-            (m_pRealStartFrame != NULL) &&
-            (m_pRealStartFrame != FRAME_TOP) &&
-            (m_pRealStartFrame->GetVTablePtr() == InlinedCallFrame::GetMethodFrameVPtr()) &&
-            (m_pThread->GetFrame() != NULL) &&
-            (m_pThread->GetFrame() != FRAME_TOP) &&
-            ((m_pThread->GetFrame()->GetVTablePtr() == ResumableFrame::GetMethodFrameVPtr()) ||
-            (m_pThread->GetFrame()->GetVTablePtr() == RedirectedThreadFrame::GetMethodFrameVPtr())));
+        if (bIsRealStartFrameUnchanged == FALSE)
+        {
+            _ASSERTE(GCStress<cfg_instr>::IsEnabled());
+            _ASSERTE(m_pRealStartFrame != NULL);
+            _ASSERTE(m_pRealStartFrame != FRAME_TOP);
+            _ASSERTE(m_pRealStartFrame->GetVTablePtr() == InlinedCallFrame::GetMethodFrameVPtr());
+            _ASSERTE(m_pThread->GetFrame() != NULL);
+            _ASSERTE(m_pThread->GetFrame() != FRAME_TOP);
+            bIsRealStartFrameUnchanged = (m_pThread->GetFrame()->GetVTablePtr() == ResumableFrame::GetMethodFrameVPtr())
+                || (m_pThread->GetFrame()->GetVTablePtr() == RedirectedThreadFrame::GetMethodFrameVPtr());
+        }
 #endif // FEATURE_HIJACK
 
         _ASSERTE(bIsRealStartFrameUnchanged);