i#1652 pcache assert: relax empty-flush pcache assert
i#1652 pcache assert: relax empty-flush pcache assert

Relax an assert on pcaches being flushed even when there is no overlap with
the flush region.

Fixes #1652

Review-URL: https://codereview.appspot.com/223020043

sigma-random/dynamorio,code4bones/dynamorio,bl4ckic3/dynamorio,code4bones/dynamorio,daksunt/dynamorio,AmesianX/dynamorio,sigma-random/dynamorio,daksunt/dynamorio,AmesianX/dynamorio,bl4ckic3/dynamorio,sigma-random/dynamorio,daksunt/dynamorio,daksunt/dynamorio,code4bones/dynamorio,code4bones/dynamorio,sigma-random/dynamorio,bl4ckic3/dynamorio,sigma-random/dynamorio,bl4ckic3/dynamorio,bl4ckic3/dynamorio,daksunt/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,code4bones/dynamorio