findDominatingStoreToReturn in CGCall.cpp didn't check if a candidate store

findDominatingStoreToReturn in CGCall.cpp didn't check if a candidate store
instruction used the ReturnValue as pointer operand or value operand. This
led to wrong code gen - in later stages (load-store elision code) the found
store and its operand would be erased, causing ReturnValue to become a <badref>.

The patch adds a check that makes sure that ReturnValue is a pointer operand of
store instruction. Regression test is also added.

This fixes PR24386.
Differential Revision: http://reviews.llvm.org/D12400

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247003 91177308-0d34-0410-b5e6-96231b3b80d8