ZJIT: Fix IsA effect

Author: tekknolagi

zjit/src/hir.rs

@@ -1165,7 +1165,8 @@ impl Insn {
             Insn::GetConstantPath { .. } => effects::Any,
             Insn::IsBlockGiven { .. } => Effect::read_write(abstract_heaps::Other, abstract_heaps::Empty),
             Insn::FixnumBitCheck { .. } => effects::Empty,
-            Insn::IsA { .. } => effects::Empty,
+            // IsA needs to read the class of the value and traverse the class hierarchy, which we model as reading from Memory.
+            Insn::IsA { .. } => Effect::read_write(abstract_heaps::Memory, abstract_heaps::Empty),
             Insn::GetGlobal { .. } => effects::Any,
             Insn::SetGlobal { .. } => effects::Any,
             Insn::GetIvar { .. } => effects::Any,