fix for ZAM optimization of assigning a record field to result of "in" operation

src/script_opt/Expr.cc

@@ -176,7 +176,6 @@ bool Expr::IsFieldAssignable(const Expr* e) const {
         case EXPR_RSHIFT:
         case EXPR_FIELD:
         case EXPR_HAS_FIELD:
-        case EXPR_IN:
         case EXPR_SIZE:
             return true;
 
@@ -189,6 +188,8 @@ bool Expr::IsFieldAssignable(const Expr* e) const {
             // case EXPR_NE:
             // case EXPR_GE:
             // case EXPR_GT:
+            //
+            // case EXPR_IN:
 
             // These could be added if we subsetted them to versions for
             // which we know it's safe to evaluate both operands.  Again

testing/btest/Baseline/opt.regress-field-assign-in-op/output

@@ -0,0 +1,2 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+nope

testing/btest/opt/regress-field-assign-in-op.zeek

@@ -0,0 +1,21 @@
+# @TEST-DOC: Regression test for assigning a record field to an "in" operation
+# @TEST-REQUIRES: test "${ZEEK_USE_CPP}" != "1"
+# @TEST-EXEC: zeek -b -O ZAM %INPUT >output
+# @TEST-EXEC: btest-diff output
+
+type r1: record {
+	is_free: bool;
+};
+
+type r2: record {
+	s: string;
+	ss: string_set;
+};
+
+event zeek_init()
+	{
+	local l1 = r1($is_free = F);
+	local l2 = r2($s="foo", $ss=set("bar", "bletch"));
+	l1$is_free = l2$s in l2$ss;
+	print l1$is_free ? "yep" : "nope";
+	}