ScopDetection: Improve printing of alias sets

We now show the all members of the alias set that may couse possible aliasing.
In case a alias set member is not a named instruction (unnamed instructions or
constant expressions), we show the expression itself.

This improves our error message

from:

  Possible aliasing for value: .reg2mem

to:

  Possible aliasing: ".reg2mem",
                     "[0 x double]* inttoptr (i64 47255179264 to [0 x double]*)

llvm-svn: 174329