Only log the visit of a return instruction if we in fact found a return

instruction.

This avoids dereferencing null in the debug logging if the instruction
was not in fact a return instruction. This potential bug was found by
PVS-Studio.

This actually fixes the last of the "dereferenced a pointer before
checking it for null" reports in the recent PVS-Studio run. However,
there are quite a few reports of this nature that I did not do anything
to fix because they are pretty glaring false positives. They usually
took the form of quite clear correlated checks or a check made in
a separate function. I've even added asserts anywhere this correlation
wasn't pretty obvious and fundamental to the code.

llvm-svn: 285988