Fix a pretty serious misfeature of the inliner: if it inlines a function

Fix a pretty serious misfeature of the inliner: if it inlines a function
with multiple return values it inserts a PHI to merge them all together.
However, if the return values are all the same, it ends up with a pointless
PHI and this pointless PHI happens to really block SRoA from happening in
at least a silly C++ example written by Doug, but probably others.  This
fixes rdar://7339069.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85206 91177308-0d34-0410-b5e6-96231b3b80d8