codegen: properly instantiate SCEVs to the place where they are used

Given the following code

    for (i = 0; i < 10; i++) {
      ;
    }

S:  A[i] = 0

When code generating S using scev based code generation, we need to retrieve
the scev of 'i' at the location of 'S'. If we do not do this the scev that
we obtain will be expressed as {0,+,1}_for and will reference loop iterators
that do not surround 'S' and that we consequently do not know how to code
generate. What we really want is the scev to be instantiated to the value of 'i'
after the loop. This value is {10} and it can be code generated without
troubles.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@177777 91177308-0d34-0410-b5e6-96231b3b80d8