don't look for SLOT_USEDUNDEF when folding :isdefined expr

`SLOT_USEDUNDEF` is such a flag that has the following meaning:
> slot has uses that might raise UndefVarError

Since `Expr(:isdefined, slot)` doesn't raise `UndefVarError`, it sounds
incorrect conceptually to look for it when folding a `:isdefined`
expression to `true`.

base/compiler/ssair/slot2ssa.jl

@@ -129,16 +129,11 @@ function fixemup!(cond, rename, ir::IRCode, ci::CodeInfo, idx::Int, @nospecializ
     if isexpr(stmt, :isdefined)
         val = stmt.args[1]
         if isa(val, Union{SlotNumber, TypedSlot})
-            slot = slot_id(val)
-            if (ci.slotflags[slot] & SLOT_USEDUNDEF) == 0
+            ssa = rename(val)
+            if ssa === UNDEF_TOKEN
+                return false
+            elseif !isa(ssa, SSAValue) && !isa(ssa, NewSSAValue)
                 return true
-            else
-                ssa = rename(val)
-                if ssa === UNDEF_TOKEN
-                    return false
-                elseif !isa(ssa, SSAValue) && !isa(ssa, NewSSAValue)
-                    return true
-                end
             end
             # temporarily corrupt the isdefined node. type_lift_pass! will fix it
             stmt.args[1] = ssa