We need to clear the pattern match map after the recursive rewrite
applications, otherwise there might be lingering entries that cause
match to fail.

Fixes #18987.

Change-Id: I7913951c455c98932bda790861db6a860ebad032
Reviewed-on: https://go-review.googlesource.com/36546
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>