Fix a minor bug in Compiler::PostVisit().

Otherwise, it unnecessarily sets foldcase for [ to ` inclusive.

Change-Id: I0d56347c97f3440d6c469ca84d4296f7d7ade540
Reviewed-on: https://code-review.googlesource.com/2475
Reviewed-by: Paul Wankadia <junyer@google.com>