Fix uninitialized read of CalleeWithThisReturn.

CalleeWithThisReturn can be left initialized if HasThisReturn() is false.
This change reverses the order of checks in EmitFunctionEpilog such that
CalleeWithThisReturn is only examined when it has a meaningful value.

Found with MemorySanitizer.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178015 91177308-0d34-0410-b5e6-96231b3b80d8