BlockGenerator: Add the possiblity to pass a set of new access functions

This change allows the BlockGenerator to be reused in contexts where we want to
provide different/modified isl_ast_expressions, which are not only changed to
a different access relation than the original statement, but which may indeed
be different for each code-generated instance of the statement.

We ensure testing of this feature by moving Polly's support to import changed
access functions through a jscop file to use the BlockGenerators support for
generating arbitary access functions if provided.

This commit should not change the behavior of Polly for now. The diff is rather
large, but most changes are due to us passing the NewAccesses hash table through
functions. This style, even though rather verbose, matches what is done
throughout the BlockGenerator with other per-statement properties.

llvm-svn: 246144