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]*)

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@174329 91177308-0d34-0410-b5e6-96231b3b80d8