Correct function to decide if a SCEV can be ignored

When doing SCEV based code generation, we ignore instructions calculating values
that are fully defined by a SCEV expression. The values that are calculated by
this instructions are recalculated on demand.

This commit improves the check to verify if certain instructions can be ignored
and recalculated on demand.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@177313 91177308-0d34-0410-b5e6-96231b3b80d8