Override operator detection logic in interactive mode

In interactive mode the token after an operator might not yet exist, so we
should disregard that info when deciding whether we see an infix operator or
not.

[Cherry-picked fca7d06]

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -997,6 +997,8 @@ object Parsers {
       leadingOperandTokens.contains(in.lookahead.token)
       || in.postfixOpsEnabled
       || in.lookahead.token == COLONop
+      || in.lookahead.token == EOF     // important for REPL completions
+      || ctx.mode.is(Mode.Interactive) // in interactive mode the next tokens might be missing
 
   /* --------- OPERAND/OPERATOR STACK --------------------------------------- */