ModuleSummaryAnalysis: Correctly handle all function operand references.

The current code that handles personality functions when creating a
module summary does not correctly handle the case where a function's
personality function operand refers to the function indirectly
(e.g. via a bitcast). This patch handles such cases by treating
personality function references like any other reference, i.e. by
adding them to the function's reference list. This has the minor side
benefit of allowing personality functions to participate in early
dead stripping.

We do this by calling findRefEdges on the function itself. This way
we also end up handling other function operands (specifically prefix
data and prologue data) for free.

Differential Revision: https://reviews.llvm.org/D37553

llvm-svn: 312698