[Scalarizer] Fix potential for stale data in Scattered across invocations

Summary:
Scalarizer has two data structures that hold information about changes
to the function, Gathered and Scattered. These are cleared in finish()
at the end of runOnFunction() if finish() detects any changes to the
function. 

However, finish() was checking for changes by only checking if
Gathered was non-empty. The function visitStore() only modifies
Scattered without touching Gathered. As a result, Scattered could have
ended up having stale data if Scalarizer only scalarized store
instructions. Since the data in Scattered is used during the execution
of the pass, this introduced dangling pointer errors. 

The fix is to check whether both Scattered and Gathered are empty
before deciding what to do in finish().

Reviewers: srhines

Reviewed By: srhines

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10422

llvm-svn: 239644