Do Not Model Unbounded Loops

  Code generation currently does not expect unbounded loops. When
  using ISL to compute the loop trip count, if we find that the
  iteration domain remains unbounded, we invalidate the Scop by
  creating an infeasible context.

Contributed-by: Matthew Simpson <mssimpso@codeaurora.org>

This fixes PR24634.

Differential Revision: http://reviews.llvm.org/D12493

llvm-svn: 246477