Constant folding should not drop distinct types

Fixes #9079

tests/distinct/t9079.nim

@@ -0,0 +1,19 @@
+discard """
+  output: '''
+25.0
+210.0
+'''
+"""
+
+type
+  Dollars = distinct float
+
+proc `$`(d: Dollars): string {.borrow.}
+proc `*` *(a, b: Dollars): Dollars {.borrow.}
+proc `+` *(a, b: Dollars): Dollars {.borrow.}
+
+var a = Dollars(20)
+a = Dollars(25.0)
+echo a
+a = 10.Dollars * (20.Dollars + 1.Dollars)
+echo a