Fix codegen for some set operations

Taking the LHS type when a temporary result value was needed lead to bad
code being generated if we get a tyRef.

Fixes #9098

tests/ccgbugs/t9098.nim

@@ -0,0 +1,12 @@
+discard """
+  targets: "c cpp js"
+  output: '''
+{'a', 'b'}
+'''
+"""
+
+var x = new(ref set[char])
+var y = new(ref set[char])
+x[] = {'a'}
+y[] = {'b'}
+echo x[] + y[]