[Fix] Non i1 typed select condition for weird pw aff functions.

  In case the pieceweise affine function used to create an isl_ast_expr
  had empty cases (e.g., with contradicting constraints on the
  parameters), it was possible that the condition of the isl_ast_expr
  select was not a comparison but a constant (thus of type i64).

  This patch does two thing:
   1) Handle the case the condition of a select is not a i1 type like C.
   2) Try to simplify the pieceweise affine functions for the min/max
      access when we generate runtime alias checks. That step can often
      remove empty or redundant cases as well as redundant constrains.

  This fixes bug: http://llvm.org/PR21167

Differential Revision: http://reviews.llvm.org/D5627